Filed pursuant to General Instruction II.L. of Form F-10
File No. 333-292019

Information contained herein is subject to completion or amendment. A registration statement relating to these securities has been filed with the Securities and Exchange Commission. This preliminary prospectus supplement and the accompanying short form base shelf prospectus shall not constitute an offer to sell or the solicitation of an offer to buy nor shall there be any sale of these securities in any jurisdiction in which such offer, solicitation or sale would be unlawful prior to registration or qualification under the securities laws of any such jurisdiction.

SUBJECT TO COMPLETION, DATED DECEMBER 11, 2025

PRELIMINARY PROSPECTUS SUPPLEMENT

(To the short form base shelf prospectus dated December 9, 2025)

US$400,000,000

   % Senior Notes due 20 

TRANSALTA CORPORATION

We are offering US$  aggregate principal amount of % senior notes due 20  (the “Notes”). The Notes will bear interest at the rate of   % per annum. Interest on the Notes will be payable semi-annually in arrears on    and    of each year, beginning on  , 2026. The Notes will mature on    , 20 . The Notes will be issued in United States dollars.

At any time on or after   , 20 , we may redeem some or all of the Notes at the redemption prices set forth in this prospectus supplement (this “Prospectus Supplement”), plus accrued and unpaid interest. Prior to    , 20 , we may redeem some or all of the Notes at a price equal to 100% of the principal amount thereof, plus accrued and unpaid interest, plus a “make-whole” premium.

In addition, prior to  , 20 , we may redeem up to 40% of the aggregate principal amount of the securities issued under the Indenture (as defined herein) in an amount not to exceed the amount of the proceeds of certain equity offerings at the redemption price set forth in this Prospectus Supplement, plus accrued and unpaid interest. We will also have the option to redeem the Notes in whole and not in part at 100% of the aggregate principal amount of the Notes, plus accrued and unpaid interest, in the event of certain changes to Canadian withholding tax laws or the enforcement or interpretation thereof. Upon the occurrence of a Change of Control Triggering Event (as hereinafter defined), holders of the Notes will have the right to require us to repurchase all or any part of their Notes at a repurchase price equal to 101% of the principal amount of the Notes, plus accrued and unpaid interest.

The Notes will be our senior unsecured obligations and will rank equally in right of payment with all of our existing and future senior indebtedness and senior in right of payment to all of our future subordinated indebtedness. The Notes will be effectively subordinated to any of our future secured indebtedness to the extent of the value of the assets securing such indebtedness. None of our existing and future subsidiaries will guarantee the Notes. As a result, the Notes will be structurally subordinated to all existing and future obligations of our subsidiaries, including trade payables and indebtedness.

The underwriters, as principals, conditionally offer the Notes, subject to prior sale, if, as and when issued by us and accepted by the underwriters in accordance with the conditions contained in the underwriting agreement referred to under “Underwriting” in this Prospectus Supplement.

Investing in the Notes involves risks. See “Risk Factors” in this Prospectus Supplement beginning on page S-22 and under the heading “Risk Factors” beginning on page 15 of the accompanying short form base shelf prospectus dated December 9, 2025 (the “Prospectus”).

Under applicable Canadian securities legislation, we may be considered to be a “connected issuer” of RBC Capital Markets, LLC, CIBC World Markets Corp., BofA Securities, Inc., Morgan Stanley & Co. LLC, Scotia Capital (USA) Inc., BMO Capital Markets Corp., TD Securities (USA) LLC, National Bank of Canada Financial Inc., ATB Securities Inc., Desjardins Securities Inc., MUFG Securities Americas Inc., J.P. Morgan Securities LLC and Mizuho Securities USA LLC, each of which is, directly or indirectly, a subsidiary or affiliate of one of our lenders to which we are currently indebted. In addition, certain of the underwriters, or their subsidiaries or affiliates, may be holders of our 7.750% senior notes due 2029 (the “2029 Notes”). We intend to use the net proceeds from this offering, together with cash on hand, to redeem all of our outstanding 2029 Notes. In addition, affiliates of RBC Capital Markets, LLC collectively own approximately 9.5% of our issued and outstanding common shares. Accordingly, the Corporation may be considered a “related issuer” or a “connected issuer” to RBC Capital Markets, LLC for purposes of applicable Canadian securities laws. See “Underwriting”.

The Notes will constitute a new issue of securities. There is no market through which Notes may be sold and purchasers may not be able to resell Notes purchased under this Prospectus Supplement. This may affect the pricing of the Notes in the secondary market, the transparency and availability of trading prices, the liquidity of the Notes, and the extent of issuer regulation. In addition, we do not intend to apply for listing of the Notes on any securities exchange. See “Risk Factors” in this Prospectus Supplement and in the Prospectus.

THE NOTES HAVE NOT BEEN APPROVED OR DISAPPROVED BY THE UNITED STATES SECURITIES AND EXCHANGE COMMISSION (THE “SEC”) OR ANY UNITED STATES STATE SECURITIES COMMISSION NOR HAS THE SEC OR ANY UNITED STATES STATE SECURITIES COMMISSION PASSED UPON THE ACCURACY OR ADEQUACY OF THIS PROSPECTUS SUPPLEMENT OR THE PROSPECTUS. ANY REPRESENTATION TO THE CONTRARY IS A CRIMINAL OFFENSE.

This Prospectus Supplement qualifies the distribution of the Notes in each of the provinces of Canada solely for the purpose of registering the Notes in the United States pursuant to the multijurisdictional disclosure system adopted by the United States. This Prospectus Supplement does not qualify the Notes for distribution to purchasers in Canada, or to residents of Canada. See “Underwriting – Offering Restrictions”. This offering is made by a Canadian issuer that is permitted, under the multijurisdictional disclosure system adopted by the United States, to prepare this Prospectus Supplement and the Prospectus in accordance with Canadian disclosure requirements. Prospective investors should be aware that such disclosure requirements are different from those of the United States. The financial statements included herein have been prepared in accordance with International Financial Reporting Standards as issued by the International Accounting Standards Board (“IFRS”). As a result, such financial statements may not be comparable to financial statements of United States companies.

Prospective investors should be aware that the acquisition of the Notes may have tax consequences both in the United States and Canada. Such tax consequences may not be described fully in this Prospectus Supplement or the Prospectus. You should read the tax discussion under “Certain Income Tax Considerations” in this Prospectus Supplement and consult with your own tax advisor with respect to your own particular circumstances.

The enforcement by investors of civil liabilities under United States federal securities laws may be affected adversely by the fact that we are incorporated and organized under the laws of Canada, that most of our officers and directors are residents of Canada, that some of the underwriters or experts named in this Prospectus Supplement, the Prospectus and the documents incorporated by reference in the Prospectus are residents of Canada, and that a substantial portion of our assets and the assets of said persons are located outside the United States.

RBC Capital Markets, LLC, on behalf of the underwriters, has advised us that the underwriters propose to offer the Notes to the public initially at the public offering price specified above. After the underwriters have made a reasonable effort to sell all of the Notes offered by this Prospectus Supplement at the price specified above, the offering price of the Notes may be decreased and may be further changed from time to time to an amount not greater than the price specified above. Any such reduction will not affect the proceeds we receive pursuant to the offering. In connection with this offering, RBC Capital Markets, LLC may purchase and sell Notes in the open market. As a result of these activities, the market price of the Notes offered under this Prospectus Supplement may be higher than the price that otherwise might exist in the open market. If these activities are commenced, they may be discontinued by RBC Capital Markets, LLC at any time without notice. See “Underwriting” in this Prospectus Supplement.

Our earnings coverage ratio on long-term debt for the twelve-month period ended September 30, 2025 is less than one to one. See “Earnings Coverage” in this Prospectus Supplement.

We expect to deliver the Notes to investors through the book-entry delivery system of The Depository Trust Company and its direct and indirect participants, including Euroclear Bank SA/NV and Clearstream Banking, S.A, against payment in New York, New York on or about , 2025.

Our head and registered office is located Suite 1400, 1100 1st Street S.E., Calgary, Alberta, T2G 1B1.

Joint Book-Running Managers

Co-Managers

IMPORTANT NOTICE ABOUT INFORMATION IN

THIS PROSPECTUS SUPPLEMENT AND THE PROSPECTUS

This document is divided into two parts. The first part is this Prospectus Supplement, which describes the specific terms of the Notes and also adds to and updates certain information contained in the Prospectus and the documents incorporated by reference into the Prospectus. The second part, the Prospectus, gives more general information, some of which may not apply to the Notes. Capitalized terms used in this Prospectus Supplement that are not defined herein have the meanings ascribed thereto in the Prospectus.

Except as set forth under “The Offering” and “Description of Notes” in this Prospectus Supplement or under “Description of Debt Securities” in the Prospectus, and unless the context otherwise requires, all references in this Prospectus Supplement to “TransAlta”, the “Corporation”, “we”, “us” and “our” mean TransAlta Corporation and its consolidated subsidiaries including any consolidated partnerships of which the Corporation or any of its subsidiaries are partners.

If the description of the Notes or any other information varies between this Prospectus Supplement, the Prospectus and the documents incorporated by reference in the Prospectus, you should rely on the information in this Prospectus Supplement.

You should rely only on the information contained in this Prospectus Supplement or contained in, or incorporated by reference into, the Prospectus or in any free writing prospectus we authorize and use in connection with the offering of the Notes. We have not, and the underwriters have not, authorized anyone to provide you with different or additional information. We are not, and the underwriters are not, making an offer to sell the Notes in any jurisdiction where the offer or sale is not permitted. You should assume that the information in this Prospectus Supplement and the Prospectus, as well as the information in any document incorporated by reference into the Prospectus previously filed with the SEC and with any securities regulatory authority in Canada, is accurate only as of the respective dates of the applicable documents. Our business, properties, financial condition, results of operations and prospects may have changed since those dates.

PRESENTATION OF FINANCIAL INFORMATION

In this Prospectus Supplement, unless otherwise specified or the context otherwise requires, all dollar amounts are expressed in Canadian dollars. “U.S. dollars” or “US$” means the lawful currency of the United States. Unless otherwise indicated, all financial information included in this Prospectus Supplement, the Prospectus and the documents incorporated by reference in the Prospectus has been prepared in accordance with IFRS. Therefore, our consolidated financial statements included in the Prospectus Supplement may not be comparable to financial statements of U.S. companies prepared in accordance with U.S. generally accepted accounting principles.

We use a number of financial measures to evaluate our performance and the performance of our business segments, including measures and ratios that do not have any standardized meaning under IFRS (collectively, “Non-IFRS Measures and Ratios”). We believe that these Non-IFRS Measures and Ratios, read together with our IFRS amounts, provide investors with a better understanding of how management assesses results. Non-IFRS Measures and Ratios are unlikely to be comparable to similar measures presented by other companies and should not be viewed in isolation from, or as an alternative for, or more meaningful than our IFRS results.

Adjusted earnings before interest, taxes, depreciation and amortization (“EBITDA”), adjusted net debt to adjusted EBITDA are some of the Non-IFRS Measures and Ratios presented in this Prospectus Supplement.

Readers should refer to the “Highlights” and “Non-IFRS and Supplementary Financial Measures” sections of our management’s discussion and analysis of financial condition and results of operations as at and for the three and nine months ended September 30, 2025 and 2024 (the “Interim MD&A”) for the composition of each Non-IFRS Measure and Ratio, the most directly comparable IFRS measure, how these Non-IFRS Measures and Ratios provide useful information to an investor, and quantitative reconciliations to the nearest applicable IFRS measures. The Interim MD&A is incorporated by reference in the Prospectus and is available under our profile on the System for Electronic Data Analysis and Retrieval+ at www.sedarplus.ca (“SEDAR+”).

TABLE OF CONTENTS

Prospectus Supplement

Exhibits

Prospectus

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EXCHANGE RATE INFORMATION

TransAlta publishes its consolidated financial information in Canadian dollars. The following table sets forth the Canada/U.S. exchange rates on the last day of the periods indicated as well as the high, low and average rates for such periods. The high, low and average exchange rates for each period were identified or calculated from spot rates in effect on each trading day during the relevant period. The exchange rates shown are expressed as the number of U.S. dollars required to purchase one Canadian dollar. These exchange rates are based on those published on the Bank of Canada’s website on each trading day (the “Bank of Canada rate”). On December 10, 2025, the Bank of Canada rate was US$0.7228 equals $1.00.

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SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS

This Prospectus Supplement, the Prospectus and the documents incorporated by reference in the Prospectus contain “forward-looking information” and “forward-looking statements” (collectively, “forward-looking statements”) within the meaning of applicable securities laws, including the “safe harbor” provisions of the Securities Act (Alberta), the United States Private Securities Litigation Reform Act of 1995, Section 21E of the United States Securities Exchange Act of 1934, as amended (the “U.S. Exchange Act”), and Section 27A of the United States Securities Act of 1933, as amended (the “Securities Act”). All forward-looking statements are based on TransAlta’s beliefs as well as assumptions based on information available at the time the assumption was made and on management’s experience and perception of historical trends, current conditions and expected further developments as well as other factors deemed appropriate in the circumstances. Forward-looking statements are not facts, but only predictions and generally can be identified by the use of statements that include phrases such as “may”, “will”, “believe”, “expect”, “anticipate”, “intend”, “plan”, “foresee”, “potential”, “enable”, “continue”, “estimate”, “would” or other words or phrases of similar import.

Forward-looking statements are subject to known and unknown risks, uncertainties and other important factors, many of which are beyond the Corporation’s control, that could cause actual events, outcomes or results to differ materially from those expressed or implied in the forward-looking statement. Although the Corporation believes that the assumptions and expectations conveyed by such forward-looking statements are reasonable based on information available on the date they are made, there can be no assurance that such assumptions and expectations will prove to be correct.

In particular, this Prospectus Supplement, the Prospectus and the documents incorporated by reference in the Prospectus contain forward-looking statements pertaining to the following: the anticipated closing of the offering of the Notes, including the timing thereof; the net proceeds from the offering and the use of such proceeds; our credit ratings outlook; the terms of the Notes; ceasing coal-fired electricity generation by the end of 2025; our greenhouse gas (“GHG”) emissions reduction targets and our carbon net-zero goal by 2045, specifically with reductions in production including lower wind resource; opportunities available to the Corporation in the Alberta market; the continued value of our ancillary services product; the benefits of the Heartland Generation acquisition; our intention to maintain a conservative credit profile; the expected completion of the Mount Keith West Network Upgrade by Q4 2025 and its anticipated average annual EBITDA; the benefits expected to arise through our pursuit of accretive opportunities with existing and prospective customers at our legacy thermal sites in Alberta and Washington State; the expected continued return to shareholders; the anticipated policy developments on Alberta’s restructured energy market by 2050, including the expected implementation timing; the transactions with Far North and PSE (each as defined herein), including expectations with respect to the timing and closing thereof, approvals required in connection therewith and the attributes of the assets acquired thereunder; our expectations regarding our revenue, expenses and operations; our expected cash needs and needs for additional financing; the anticipated trends and challenges in our business and the markets in which we operate; our expectation to continue maintaining adequate liquidity; and our expectation that the redemption of the 2029 Notes will be funded with the net proceeds of this offering, together with cash on hand.

The forward-looking statements contained in this Prospectus Supplement, the Prospectus and the documents incorporated by reference in the Prospectus are based on many assumptions including, but not limited to, the following: our ability to complete the offering on a timely basis and on the terms expected; fulfillment by the underwriters of their obligations pursuant to the underwriting agreement; that no event will occur which would allow the underwriters to terminate their obligations under the underwriting agreement; no significant changes to applicable laws and regulations beyond those that have already been announced; no significant changes to fuel and purchased power costs; no material adverse impacts to long-term investment and credit markets; no significant changes to power price and hedging assumptions, including hedged volumes and prices; no significant changes to gas commodity prices and transport costs; no significant changes to decommissioning and restoration costs; no significant changes to interest rates; no significant changes to the demand and growth of renewables generation; no significant changes to the integrity and reliability of our assets; planned and unplanned

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outages and use of our assets; and no significant changes to the Corporation’s debt and credit ratings. Although the Corporation believes that these assumptions are reasonable based on currently available information, there can be no assurance that such assumptions will prove to be correct.

Forward-looking statements are subject to a number of significant risks and uncertainties that could cause actual plans, performance, results or outcomes to differ materially from current expectations. Factors that may adversely impact what is expressed or implied by forward-looking statements contained in this Prospectus Supplement, the Prospectus and the documents incorporated by reference in the Prospectus include risks relating to: fluctuations in power prices; changes in supply and demand for electricity; our ability to contract our electricity generation for prices that will provide expected returns; our ability to replace contracts as they expire; risks associated with development projects and acquisitions; any difficulty raising needed capital in the future on reasonable terms or at all; our ability to achieve our targets relating to environment, social and governance (“ESG”); long-term commitments on gas transportation capacity that may not be fully utilized over time; changes to the legislative, regulatory and political environments; environmental requirements and changes in, or liabilities under, these requirements; operational risks involving our facilities, including unplanned outages and equipment failure; disruptions in the transmission and distribution of electricity; reductions in production; impairments and/or writedowns of assets; adverse impacts on our information technology systems and our internal control systems, including increased cybersecurity threats; commodity risk management and energy trading risks; reduced labour availability and ability to continue to staff our operations and facilities; disruptions to our supply chains; climate-change related risks; reductions to our generating units’ relative efficiency or capacity factors; general economic risks, including deterioration of equity and debt markets, increasing interest rates or rising inflation; general domestic and international economic and political developments, including potential trade tariffs; industry risk and competition; counterparty credit risk; inadequacy or unavailability of insurance coverage; increases in the Corporation’s income taxes and any risk of reassessments; legal, regulatory and contractual disputes and proceedings involving the Corporation; reliance on key personnel; and labour relations matters. Additional information about material factors that could cause actual results to differ materially from expectations and about material factors or assumptions applied in making forward-looking statements may be found in this Prospectus Supplement and the Prospectus under “Risk Factors” as well as in our management’s discussion and analysis of financial condition and results of operations as of and for the year ended December 31, 2024 (the “Annual MD&A”) and the Annual Information Form (as defined herein), and elsewhere in TransAlta’s filings with the Canadian and U.S. securities regulators.

Readers are urged to consider these factors carefully in evaluating the forward-looking statements and are cautioned not to place undue reliance on these forward-looking statements. The forward-looking statements included in this Prospectus Supplement, the Prospectus and the documents incorporated by reference in the Prospectus are made only as of the date such statements and the Corporation does not undertake to publicly update these forward-looking statements to reflect new information, future events or otherwise, except as required by applicable laws. The purpose of the financial outlooks contained herein is to give the reader information about management’s current expectations and plans and readers are cautioned that such information may not be appropriate for other purposes. In light of these risks, uncertainties and assumptions, the forward-looking statements might occur to a different extent or at a different time than we have described, or might not occur at all. We cannot assure that projected results or events will be achieved.

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DOCUMENTS INCORPORATED BY REFERENCE

As of the date of this Prospectus Supplement, the following documents filed with the securities commissions or similar authorities in each of the provinces of Canada and with the SEC are specifically incorporated by reference into the Prospectus:

This Prospectus Supplement is deemed to be incorporated by reference into the Prospectus solely for the purposes of the offering of Notes.

Any documents of the type required to be incorporated by reference in a short form prospectus pursuant to National Instrument 44-101 — Short Form Prospectus Distributions of the Canadian Securities Administrators, including any documents of the type referred to above or under “Documents Incorporated by Reference” in the Prospectus, material change reports (excluding confidential material change reports) and business acquisition reports we subsequently file with any securities commissions or similar authorities in Canada after the date of this Prospectus Supplement and prior to the termination of any offering of the Notes under this Prospectus Supplement shall be deemed to be incorporated by reference into the Prospectus. These documents are available through the internet on SEDAR+, which can be accessed at www.sedarplus.ca. In addition, any similar documents we file with, or furnish to, the SEC pursuant to Section 13(a), 13(c) or 15(d) of the U.S. Exchange Act on or after the date of this Prospectus Supplement and prior to the termination of this distribution of Notes, shall be deemed to be incorporated by reference into the Prospectus and the registration statement on Form F-10 of which this Prospectus Supplement and the Prospectus form a part, if and to the extent expressly provided in such report, except that any report on Form 6-K shall be incorporated only to the extent expressly provided in such report. To the extent that any document incorporated by reference into the Prospectus is included in a report that is filed with or furnished to the SEC by us on Form 40-F, 20-F, 10-K, 10-Q, 8-K or 6-K (or any respective successor form), such document shall also be deemed to be incorporated by reference as an exhibit to the registration statement on Form F-10 of which this Prospectus Supplement and the Prospectus form a part. Our reports on Form 6-K, and our annual reports on Form 40-F, are available on the SEC’s website at www.sec.gov.

Any statement contained in this Prospectus Supplement, the Prospectus, or in a document incorporated or deemed to be incorporated by reference therein, shall be deemed to be modified or superseded for the purposes of this Prospectus Supplement to the extent that a statement contained herein or therein or in any other subsequently filed document that also is or is deemed to be incorporated by reference therein modifies or supersedes such statement. The modifying or superseding statement need not state that it has modified or superseded a prior statement or include any other information set forth in the document that it modifies or supersedes. The making of a modifying or superseding statement is not to be deemed an admission for any purposes that the modified or superseded statement, when made, constituted a misrepresentation, an untrue statement of a material fact or an omission to state a material fact that is required to be stated or that is necessary to make a statement not misleading in light of the circumstances in which it was made. Any statement so modified or superseded shall not be deemed, except as so modified or superseded, to constitute a part of this Prospectus Supplement.

We will provide without charge to each person to whom this Prospectus Supplement is delivered, including any beneficial owner, upon written or oral request of such person, a copy of any or all of the documents incorporated by reference in the Prospectus (other than exhibits to such documents, unless such exhibits are specifically incorporated by reference in such documents). Requests should be directed to TransAlta Corporation, TransAlta at Suite 1400, 1100 1st Street S.E., Calgary, Alberta, T2G 1B1, Attention: Corporate Secretary, telephone number (403) 267-7110. These documents are also available on SEDAR+ at www.sedarplus.ca and on EDGAR at www.sec.gov.

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WHERE YOU CAN FIND MORE INFORMATION

We have filed with the SEC under the U.S. Securities Act a registration statement on Form F-10 relating to the Notes and of which this Prospectus Supplement and the Prospectus form a part. This Prospectus Supplement and the Prospectus do not contain all of the information set forth in such registration statement, certain items of which are contained in the exhibits to such registration statement as permitted or required by the rules and regulations of the SEC. See “Documents Filed as Part of the Registration Statement” in the Prospectus. Statements made in this Prospectus Supplement and the Prospectus as to the contents of any contract, agreement or other document referred to are not necessarily complete, and in each instance, reference is made to the exhibit, if applicable, for a more complete description of the relevant matter, each such statement being qualified in its entirety by such reference. Items of information omitted from this Prospectus Supplement and the Prospectus but contained in the registration statement on Form F-10 are available on the SEC’s website at www.sec.gov.

We are subject to the information requirements of the U.S. Exchange Act, and, in accordance therewith, file reports and other information with the SEC. Under the multijurisdictional disclosure system adopted in the United States, such reports and other information, subject to certain exceptions, may be prepared in accordance with the disclosure requirements of Canada, which requirements are different from those of the United States. We are exempt from the rules under the U.S. Exchange Act prescribing the furnishing and content of proxy statements, and our officers, directors and principal shareholders are exempt from the reporting and short swing profit recovery provisions contained in Section 16 of the U.S. Exchange Act. Under the U.S. Exchange Act, we are not required to publish financial statements as promptly as United States companies. Such reports and other information are available on the SEC’s EDGAR website at www.sec.gov. Prospective investors may read and download any public document that we have filed with the securities regulatory authorities in each of the provinces of Canada on SEDAR+ at www.sedarplus.ca.

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THE OFFERING

The following is a brief summary of some of the terms of this offering. For a more complete description of the terms of the Notes, see “Description of Notes” in this Prospectus Supplement and “Description of Debt Securities” in the Prospectus. In this section, “we”, “us” and “our” refer only to TransAlta Corporation and not to any of its subsidiaries, unless otherwise stated.

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RISK FACTORS

An investment in the Notes is subject to a number of risks. In addition to the other information contained in this Prospectus Supplement, the Prospectus and the documents incorporated by reference in the Prospectus, you should consider carefully the risk factors set forth below, as well as those set forth under the heading “Risk Factors” in the Prospectus and under the headings “Risk Factors” and “Governance and Risk Management” in the Annual MD&A. The risks and uncertainties described below, in the Prospectus and in the documents incorporated by reference in the Prospectus are not the only ones applicable to an investment in the Notes. Additional risks and uncertainties that we are unaware of, or that we currently deem to be immaterial, may also become important factors that affect an investment in the Notes. If any of the following risks actually occurs, our business, financial condition or results of operations could be materially adversely affected, with the result that the trading price of the Notes could decline and you could lose all or part of your investment.

There is no public market for the Notes.

The Notes are a new issue of securities for which there is currently no public market. We do not intend to apply for listing of the Notes on any securities exchange or to arrange for the Notes to be quoted on any automated dealer quotation system. If the Notes are traded after their initial issue, they may trade at a discount from their initial offering prices, depending on prevailing interest rates, the market for similar securities and other factors, including general economic conditions and our financial condition. We cannot assure you as to the liquidity of the trading market for the Notes or that a trading market for the Notes will develop.

Changes in interest rates may cause the market value of the Notes to decline.

Prevailing interest rates will affect the market price or value of the Notes. The market price or value of the Notes may decline as prevailing interest rates for comparable debt instruments rise, and increase as prevailing interest rates for comparable debt securities decline.

Changes in our credit rating or outlook or in the rating assigned by a rating agency to the Notes could adversely affect the market price or liquidity of the Notes.

Credit rating agencies continually revise their ratings and outlook for the companies that they follow, including us. The credit rating agencies also evaluate our industry as a whole and may change their credit ratings or outlook for us based on their overall view of our industry. We cannot be sure that credit rating agencies will maintain their ratings on the Notes. A negative change in our ratings or outlook could have an adverse effect on the market value of the Notes.

We expect that the Notes will be rated by nationally recognized statistical rating agencies. We cannot assure you that any rating assigned will remain for any given period of time or that a rating will not be lowered or withdrawn entirely by a rating agency if, in that rating agency’s judgment, circumstances relating to the basis of the rating, such as adverse changes in our business, so warrant. Any lowering or withdrawal of a rating by a rating agency could reduce the liquidity or market value of the Notes.

The Notes are unsecured obligations of the Corporation.

The Notes will be our direct unsecured obligations, ranking equally and pari passu, except as to sinking fund or analogous provisions, with all of our other unsecured and unsubordinated indebtedness. The Notes will be structurally subordinated to all indebtedness and other liabilities of our subsidiaries and will be effectively subordinated to all of our existing and future secured indebtedness, to the extent of the value of the assets securing such secured indebtedness. If we are involved in any bankruptcy, dissolution, liquidation or reorganization, the holders of indebtedness and liabilities of our subsidiaries would be paid before the holders of Notes receive any amounts due under the Notes and the holders of our secured indebtedness would be paid

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before the holders of Notes receive any amounts due under the Notes, to the extent of the value of the assets securing such secured indebtedness. In that event, a holder of Notes may not be able to recover any principal or interest due under the Notes.

We may not be able to fulfill our repurchase obligations with respect to the Notes upon a change of control.

If we experience a Change of Control Triggering Event, we will be required to make an offer to repurchase all outstanding Notes at a repurchase price equal to 101% of the principal amount of the Notes repurchased, plus accrued and unpaid interest, if any, to, but not including, the applicable repurchase date. Failure to repurchase, or to make an offer to repurchase, the Notes would constitute a default under the Indenture governing the Notes, which would also constitute a default under certain instruments governing our existing indebtedness. See “Description of Notes – Repurchase Upon Change of Control Triggering Event” in this Prospectus Supplement.

If a Change of Control Triggering Event were to occur, we cannot assure you that we would have sufficient funds to repay any Notes that we would be required to offer to repurchase, or to satisfy any other obligations that would become immediately due and payable under the other instruments governing our indebtedness, as a result of such Change of Control Triggering Event. In order to satisfy our obligations, we may attempt to refinance our indebtedness or obtain consents from our other lenders or from the holders of the Notes. We cannot assure you that we would be able to refinance our indebtedness or obtain such consents on satisfactory terms or at all.

Because the Indenture governing the Notes will not contain limits on the amount of additional debt that we may incur, our ability to make timely payments on the Notes may be adversely affected by the amount and terms of our future debt.

Our ability to make timely payments on our outstanding debt may depend on the amount and terms of our other obligations, including any other senior debt securities issued by the Corporation. The Indenture governing the Notes will not contain any limitation on the amount of indebtedness or other liabilities that we or any of our subsidiaries may incur in the future, including additional senior debt securities. In the event we issue additional notes under the Indenture or incur other indebtedness, unless our earnings grow in proportion to our debt and other fixed charges, our ability to service the Notes on a timely basis may become impaired. We expect that we will from time to time incur additional debt and other liabilities. In addition, TransAlta will not be restricted from paying dividends on or repurchasing its securities under the Indenture governing the Notes.

We may redeem the Notes before they mature, which could occur when prevailing interest rates are relatively low.

We may redeem all or any portion of the Notes at our option as described under “Description of Notes – Optional Redemption.” Any such redemption may occur when prevailing interest rates are lower than the rate borne by the Notes. These redemption rights may, depending on prevailing market conditions at the time, create reinvestment risk for the noteholders in that they may be unable to find a suitable replacement investment with a comparable return to those Notes. If prevailing rates are lower at the time of redemption, noteholders may not be able to reinvest the redemption proceeds in a comparable security at an effective interest rate as high as the interest rate on the Notes being redeemed. The redemption of the Notes also may adversely affect noteholders’ ability to sell the Notes if and at any time after the Notes are called for partial or full redemption.

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USE OF PROCEEDS

We expect that the net proceeds from this offering will be approximately US$    million after deducting underwriting commissions and estimated expenses of this offering. We intend to use the net proceeds from this offering, together with cash on hand, to redeem the 2029 Notes.

The underwriting commission will be paid by the Corporation from the gross proceeds of the offering of Notes. The expenses of the offering will be paid from the general funds of the Corporation.

The Corporation’s overall corporate strategy and major initiatives supporting its strategy are summarized in the Annual MD&A and the Annual Information Form.

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PRIOR SALES

The Corporation has not sold or issued any U.S. dollar denominated senior notes, or securities convertible into U.S. dollar denominated senior notes, during the 12-month period prior to the date hereof.

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CAPITALIZATION

The following table sets forth TransAlta’s debt capitalization as at September 30, 2025. TransAlta’s debt capitalization is presented on an actual basis, and on an as adjusted basis, after giving effect to this offering and the application of the net proceeds therefrom as described under “Use of Proceeds”. As noted under “Use of Proceeds”, we intend to use the net proceeds from this offering, together with cash on hand, to redeem the 2029 Notes. As a result, it is expected that this offering will not result in an increase in our long-term debt except on account of discounts, commissions and other expenses of this offering. You should read this table together with our unaudited consolidated financial statements for the three and nine months ended September 30, 2025 which are included in this Prospectus Supplement. All U.S. dollar amounts in the following table have been converted to Canadian dollars using the exchange rate of US$1.00 = $1.3922.

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DESCRIPTION OF NOTES

The following description of the terms of the Notes supplements, and to the extent inconsistent therewith replaces, the description set forth under the heading “Description of Debt Securities” in the Prospectus and should be read in conjunction with such description. In this section, “Corporation” refers only to TransAlta Corporation and not to any of its subsidiaries, unless otherwise stated. All capitalized terms used under this heading “Description of Notes” that are not defined herein have the meanings ascribed thereto in the Prospectus.

General

The Notes will be issued under an indenture dated as of June 25, 2002 between TransAlta and The Bank of New York Mellon (formerly known as The Bank of New York) as trustee (the “Trustee”), as supplemented by a supplemental indenture to be dated as of the issue date of the Notes (as so supplemented, the “Indenture”). The Indenture is subject to, and governed by, the United States Trust Indenture Act of 1939, as amended. The following summary of certain provisions of the Indenture and the Notes does not purport to be complete and is qualified in its entirety by reference to the actual provisions of the Indenture. A copy of the Indenture will be available on SEDAR+ at www.sedarplus.ca and on EDGAR at www.sec.gov.

The Notes will be direct unsecured obligations of the Corporation and will rank equally and ratably with all other unsubordinated and unsecured indebtedness of the Corporation.

Payment of the principal, premium, if any, and interest on the Notes will be made in U.S. dollars.

Form and Denominations

The Notes will be issuable in minimum denominations of US$2,000 or integral multiples of US$1,000 in excess thereof. The Notes will be represented by one or more fully registered global notes deposited in book-entry form with DTC, New York, New York, and registered in the name of Cede & Co. (DTC’s partnership nominee) or such other name as may be requested by an authorized representative of DTC.

The Notes will initially be issued in an aggregate principal amount of US$400,000,000 and will mature on     , 20 . The Notes will bear interest at the rate of   % per annum from     , 2025 or from the most recent date to which interest has been paid or provided for, payable semi-annually on      and      of each year, commencing     , 2026, to the persons in whose names the Notes are registered at the close of business on the preceding      or     , respectively. Interest shall be computed assuming a 360-day year consisting of twelve 30-day months.

The Corporation may from time to time, without the consent of the holders of the Notes, issue additional Notes after this offering. The Notes and any additional Notes subsequently issued under the Indenture will be treated as a single class for all purposes under the Indenture (except in respect of the payment of interest accruing prior to the issue date of the additional Notes and the first payment of interest following the issue date of the additional Notes), including, without limitation, waivers, amendments, redemptions and offers to purchase.

The Notes will not be entitled to the benefits of any sinking fund.

Optional Redemption

On and after     , 20 , the Corporation may, at its option on one or more occasions, redeem all or a part of the Notes, upon notice as described under “– Selection and Notice”, at the redemption prices (expressed as percentages of principal amount) set forth below, plus accrued and unpaid interest, if any, on the Notes to be redeemed to, but not including, the applicable redemption date (subject to the right of holders of record on the

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relevant record date to receive interest due on an interest payment date that is on or prior to the redemption date), if redeemed during the twelve-month period beginning on     of the years indicated below:

At any time prior to     , 20 , the Corporation may, at its option on one or more occasions redeem up to 40% of the aggregate principal amount of the securities issued under the Indenture, upon notice as described under “– Selection and Notice”, at a redemption price of   % of the principal amount, plus accrued and unpaid interest, if any, to, but not including, the applicable redemption date (subject to the right of holders of record on the relevant record date to receive interest due on an interest payment date that is on or prior to the redemption date), with an amount of cash not greater than the net cash proceeds of one or more Equity Offerings by the Corporation, provided, that for purposes of calculating the principal amount of the Notes able to be redeemed with such cash proceeds of such Equity Offering or Equity Offerings, such amount shall include only the principal amount of the Notes to be redeemed plus the premium on such Notes to be redeemed, provided further that:

(1)  at least 60% of the aggregate principal amount of the securities issued under the Indenture remains outstanding immediately after the occurrence of such redemption (excluding any such securities held by the Corporation and its Subsidiaries); and

(2)  the redemption occurs within 180 days of the date of the closing of the related Equity Offering.

In addition, at any time prior to     , 20 , the Corporation may, at its option on one or more occasions, redeem all or a part of the Notes, upon notice as described under “– Selection and Notice”, at a redemption price equal to the sum of:

(1)  100% of the principal amount thereof, and

(2)  the Make Whole Premium (as defined herein) as of the applicable redemption date,

plus accrued and unpaid interest, if any, to, but not including, the applicable redemption date (subject to the right of holders of record on the relevant record date to receive interest due on an interest payment date that is on or prior to the redemption date).

“Business Day” means each day that is not a Saturday, Sunday or other day on which banking institutions in New York, New York, Calgary, Alberta or another place of payment are authorized or required by law to close.

“Equity Offering” means any public or private sale of capital stock of the Corporation made on a primary basis by the Corporation (other than (a) capital stock that is mandatorily redeemable or otherwise required to be repurchased at the option of the holder thereof on or prior to the date that is 91 days after the date on which the Notes mature and (b) any sale to a Subsidiary of the Corporation).

“Make Whole Premium” means, with respect to a Note as of any redemption date for such Note whose redemption price may be determined by reference to the Make Whole Premium, the excess, if any, of (1) the present value as of the applicable redemption date of (a) the redemption price of such Note at     , 20  (such redemption price being set forth in the first full paragraph of this “– Optional Redemption” section) plus (b) any required interest payments due on such Note through     , 20  (except for accrued and unpaid interest to, but not including, the applicable redemption date), computed using a discount rate equal to the Treasury Rate plus  basis points, discounted to the redemption date on a semi-annual basis (assuming a 360-day year consisting of twelve 30-day months), over (2) the then-outstanding principal amount of such Note.

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“Treasury Rate” means, with respect to any redemption date for any Note whose redemption price may be determined by reference to the Make Whole Premium, the yield to maturity as of the redemption date of United States Treasury securities with a constant maturity (as compiled and published in the most recent Selected Interest Rates (Daily)–H.15 that has become publicly available at least two Business Days prior to such date (or, if such statistical release is no longer published, any publicly available source of similar market data selected by the Corporation)) most nearly equal to the period from such date to     , 20 ; provided, that if such period is not equal to the constant maturity of a United States Treasury security for which a weekly average yield is given, the Corporation shall obtain the Treasury Rate by linear interpolation (calculated to the nearest one-twelfth of a year) from the weekly average yields of United States Treasury securities for which such yields are given, except that if the period from the redemption date to     , 20  is less than one year, the weekly average yield on actually traded United States Treasury securities adjusted to a constant maturity of one year shall be used. Calculation of the Make Whole Premium and the Treasury Rate will be made by the Corporation or on behalf of the Corporation by such Person as the Corporation shall designate. The Corporation will (1) calculate the Treasury Rate and the Make Whole Premium no later than the first (and no earlier than the fourth) Business Day preceding the applicable redemption date (or, in the case of any redemption in connection with a defeasance of the Notes or a satisfaction and discharge of the Indenture, on the Business Day preceding such event), and (2) prior to such redemption date (or such event, as applicable), file with the Trustee a statement setting forth the Treasury Rate and the Make Whole Premium and showing the calculation of each in reasonable detail.

Repurchase Upon Change of Control Triggering Event

If a Change of Control Triggering Event occurs, unless the Corporation has exercised its right to redeem the Notes as described under “– Optional Redemption”, each holder of Notes will have the right to require the Corporation to purchase all or a portion of such holder’s Notes pursuant to the offer described below (the “Change of Control Offer”). In the Change of Control Offer, the Corporation will offer a payment (the “Change of Control Payment”) equal to 101% of the principal amount thereof, plus accrued and unpaid interest, if any, thereon to, but not including, the date of purchase, subject to the rights of holders of Notes on the relevant record date to receive interest due on the relevant interest payment date.

Within 30 days following the date upon which the Change of Control Triggering Event occurred, or at the Corporation’s option, prior to any Change of Control but after the public announcement of the pending Change of Control, the Corporation will be required to send, by first class mail or otherwise deliver (including by electronic delivery), a notice to each holder of Notes, with a copy to the Trustee, which notice will govern the terms of the Change of Control Offer. Such notice will state, among other things, the purchase date, which must be no earlier than 10 days nor later than 60 days from the date such notice is mailed or otherwise delivered (including by electronic delivery), other than as may be required by law (the “Change of Control Payment Date”). The notice, if mailed or otherwise delivered (including by electronic delivery) prior to the date of consummation of the Change of Control, will state that the Change of Control Offer is conditional on the Change of Control being consummated on or prior to the Change of Control Payment Date. Each holder of Notes electing to have Notes purchased pursuant to a Change of Control Offer will be required to surrender their Notes, with the form entitled “Option of Holder to Elect Purchase” on the reverse of the Note completed, to the paying agent at the address specified in the notice, or transfer their Notes to the paying agent by book-entry transfer pursuant to the applicable procedures of the paying agent, prior to the close of business on the third business day prior to the Change of Control Payment Date.

The Corporation will not be required to make a Change of Control Offer if (a) a third party makes such an offer in the manner, at the times and otherwise in compliance with the requirements for such an offer made by the Corporation and such third party purchases all Notes properly tendered and not withdrawn under its offer; (b) a notice of redemption has been given, unless and until there is a default in payment of the applicable redemption price or (c) in connection with or in contemplation of any Change of Control, the Corporation or a third party has made an offer to purchase (an “Alternate Offer”) any and all Notes validly tendered at a cash price equal to or higher than the Change of Control Payment and has purchased all Notes properly tendered in accordance with the terms of such Alternate Offer. Notwithstanding anything to the contrary contained herein, a Change of Control Offer, tender offer or Alternate Offer by the Corporation or a third party may be made in

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advance of a Change of Control Triggering Event or Change of Control, conditioned upon the occurrence of such Change of Control Triggering Event or a Change of Control, if a definitive agreement is in place for the Change of Control at the time the Change of Control Offer, tender offer or Alternate Offer is made.

In connection with any Change of Control Offer, tender offer or Alternate Offer by the Corporation or any third party to purchase all of the Notes, if holders of not less than 90.0% of the aggregate principal amount of the then outstanding Notes validly tender and do not validly withdraw such Notes in connection with such Change of Control Offer, tender offer or Alternate Offer and the Corporation or such third party purchases all of the Notes validly tendered and not validly withdrawn by such holders, all of the holders of the Notes will be deemed to have consented to such Change of Control Offer, tender offer or Alternate Offer and accordingly, the Corporation or such third party (as applicable) will have the right upon not less than 10 days’ nor more than 60 days’ prior written notice, given not more than 60 days following such purchase date pursuant to the Change of Control Offer, tender offer or Alternate Offer, to purchase all Notes that remain outstanding following such purchase at a purchase price equal to the highest price offered to each other holder in such Change of Control Offer, tender offer or Alternate Offer, plus, to the extent not included in the Change of Control Offer, tender offer or Alternate Offer, accrued and unpaid interest to, but excluding, the applicable purchase date (subject to the right of the holders of record on the relevant record date to receive interest due on an interest payment date that is on or prior to the applicable purchase date).

The definition of Change of Control includes a phrase relating to the direct or indirect sale, transfer, conveyance or other disposition of “all or substantially all” of the assets of the Corporation and its subsidiaries taken as a whole. Although there is a limited body of case law interpreting the phrase “substantially all”, there is no precise established definition of the phrase under applicable law. Accordingly, the ability of a holder of Notes to require the Corporation to repurchase its Notes as a result of a sale, transfer, conveyance or other disposition of less than all of the assets of the Corporation and its subsidiaries taken as a whole to another “person” may be uncertain. In addition, a Delaware Chancery Court decision raised questions about the enforceability of provisions, which are similar to those in the Indenture, related to the triggering of a change of control as a result of a change in the composition of a board of directors. Accordingly, the ability of a holder of Notes to require the Corporation to repurchase its Notes as a result of a change in the composition of the board of directors of the Corporation may be uncertain.

To the extent that the provisions of any applicable securities laws or regulations conflict with the Change of Control provisions of the Indenture, the Corporation will comply with the applicable securities laws and regulations and will not be deemed to have breached its obligations under the Change of Control provisions of the Indenture by virtue of such compliance.

“Ratings Decline” means the occurrence of a decrease in the rating of the Notes by one or more gradations (including gradations within the rating categories, as well as between categories) by each of the Rating Agencies, within 60 days of the earliest of (a) a Change of Control, (b) the date of public notice of the occurrence of a Change of Control or (c) public notice of the intention of the Corporation to effect a Change of Control (which 60-day period shall be extended so long as the rating of the Notes is under publicly announced consideration for possible downgrade by a Rating Agency); provided, however, that notwithstanding the foregoing, a Ratings Decline shall be deemed not to have occurred if any of the Rating Agencies rates the Notes with an Investment Grade Rating that is not subject to review for possible downgrade on such 60^th day.

The Trustee shall not be charged with knowledge of, or be responsible for the monitoring of, the ratings of the Notes.

“Change of Control” means the occurrence of any of the following:

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Notwithstanding the foregoing, (1) a transaction will not be deemed to involve a Change of Control under clause (b) above if (i) we become a direct or indirect wholly-owned subsidiary of a holding company and (ii) (A) the direct or indirect holders of the Voting Shares of such holding company immediately following that transaction are substantially the same as the holders of the Corporation’s Voting Shares immediately prior to that transaction or (B) immediately following that transaction no person (other than a holding company satisfying the requirements of this sentence) is the beneficial owner, directly or indirectly, of more than 50% of the Voting Shares of such holding company; and (2) the term “Change of Control” shall not include a merger, amalgamation or consolidation of the Corporation with, or the sale, lease, transfer, conveyance or other disposition of all or substantially all of the assets of the Corporation and its subsidiaries taken as a whole to, an affiliate of the Corporation that is incorporated or organized solely for the purpose of reincorporating or reorganizing the Corporation in another jurisdiction, which is not otherwise prohibited by the terms of the Indenture. The term “person”, as used in this definition, has the meaning given thereto in Section 13(d)(3) of the U.S. Exchange Act.

“Change of Control Triggering Event” means the occurrence of both a Change of Control and so long as the Notes are rated, a related Ratings Decline.

“Investment Grade Rating” means a rating equal to or higher than Baa3 (or the equivalent) by Moody’s and BBB- (or the equivalent) by S&P, or, in each case, if such Rating Agency ceases to make a rating of the Notes publicly available, the equivalent investment grade credit rating by the replacement agency selected by the Corporation in accordance with the procedures described below.

“Rating Agencies” means Moody’s and S&P; and if either ceases to make a rating of the Notes publicly available, a “nationally recognized statistical rating organization”, within the meaning of Section 3(a)(62) under the U.S. Exchange Act, selected by the Corporation (as certified by a resolution of the Corporation’s board of directors) as a replacement agency for Moody’s or S&P, or each of them, as the case may be.

“Voting Shares” means, with respect to any specified person as of any date, the shares of such person that are at the time entitled to vote generally in the election of the board of directors of such person.

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Tax Redemption

The Notes will be subject to redemption at any time, in whole but not in part, at the option of the Corporation, at a redemption price equal to the principal amount thereof together with accrued and unpaid interest to the date fixed for redemption, upon the giving of a notice as described below in “– Selection and Notice”, if (1) the Corporation determines that (a) as a result of any change in or amendment to the laws (or any regulations or rulings promulgated thereunder) of Canada or of any political subdivision or taxing authority thereof or therein affecting taxation, or any change in official position regarding application or interpretation of such laws, regulations or rulings (including a holding by a court of competent jurisdiction), which change or amendment is announced or becomes effective on or after the date of this Prospectus Supplement, the Corporation has or will become obligated to pay, on the next succeeding date on which interest is due, additional amounts with respect to the Notes as described under “Description of Debt Securities – Payment of Additional Amounts” in the Prospectus; or (b) on or after the date of this Prospectus Supplement, any action has been taken by any taxing authority of, or any decision has been rendered by a court of competent jurisdiction in, Canada or any political subdivision or taxing authority thereof or therein, including any of those actions specified in (a) above, whether or not such action was taken or decision was rendered with respect to the Corporation, or any change, amendment, application or interpretation shall be officially proposed, which, in any such case, in the written opinion to the Corporation of legal counsel of recognized standing, will result in the Corporation becoming obligated to pay, on the next succeeding date on which interest is due, additional amounts with respect to the Notes and (2) in any such case, the Corporation in its business judgment determines that such obligation cannot be avoided by the use of reasonable measures available to the Corporation; provided, however, that (i) no such notice of redemption may be given earlier than 60 days prior to the earliest date on which the Corporation would be obligated to pay such additional amounts were a payment in respect of the Notes then due, and (ii) at the time such notice of redemption is given, such obligation to pay such additional amounts remains in effect; and provided, further, that any such notice of redemption shall be given no later than 30 days prior to such redemption.

In the event that the Corporation elects to redeem the Notes pursuant to the provisions set forth in the preceding paragraph, the Corporation shall deliver to the Trustee a certificate, signed by an authorized officer, stating that the Corporation is entitled to redeem the Notes pursuant to their terms.

Prior to the publication or, where relevant, mailing of any notice of redemption of the Notes pursuant to the foregoing, the Corporation will deliver the Trustee an opinion of counsel to the effect that there has been such change or amendment which would entitle the Corporation to redeem the Notes hereunder. In addition, before the Corporation publishes or mails notice of redemption of the Notes as described above, it will deliver to the Trustee a certificate, signed by an authorized officer, stating that the Corporation cannot avoid its obligation to pay additional amounts by taking reasonable measures available to it and all other conditions for such redemption have been met.

The Trustee shall be entitled to rely on such officer’s certificate and opinion of counsel as sufficient evidence of the existence and satisfaction of the conditions precedent as described above, in which event it will be conclusive and binding on the holders of the Notes. The Trustee shall not be responsible for the determination of any redemption price.

Selection and Notice

Notice of any redemption will be mailed by first class mail (or sent electronically if DTC is the recipient) at least 10 days but not more than 60 days before the redemption date to each holder of Notes to be redeemed at its registered address. No Notes of US$2,000 or less can be redeemed in part.

Notes called for redemption will become due on the date fixed for redemption, subject to the Corporation’s right to delay or rescind an optional redemption date as provided below. Unless the Corporation

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defaults in payment of the redemption price, on and after the redemption date, interest will cease to accrue on the Notes or portions thereof called for redemption. If fewer than all of the Notes of the applicable series are to be redeemed, the particular Notes or portions thereof to be redeemed will be selected, not more than 60 days prior to the redemption date, from the outstanding Notes of such series not previously called (a) if such Notes are held in global form, in accordance with the procedures of DTC or (b) if such Notes are held in certificated form, on a pro rata basis.

Notice of any optional redemption of the Notes may, at the Corporation’s discretion, be subject to one or more conditions precedent, including, but not limited to, (i) the completion of one or more Equity Offerings or other securities offerings or other financings or the completion of any transaction (or series of related transactions) that constitutes a Change of Control; and (ii) any other instructions, as determined by the Corporation, that a holder of Notes must follow. If an optional redemption of the Notes is subject to satisfaction of one or more conditions precedent, such notice may state that, at the Corporation’s discretion, the redemption date may be delayed on one or more occasions either to a date specified in a subsequent notice to holders of the Notes or until such time (which date or time may be more than 60 days after the date the notice of redemption was mailed or otherwise sent) as any or all such conditions shall be satisfied or waived, and that such redemption will not occur and such notice will be rescinded if any or all such conditions shall not have been satisfied as and when required (as determined by the Corporation in its sole discretion taking into account any election by the Corporation to delay such redemption date), unless the Corporation has waived any such conditions that are not satisfied, or at any time if in the good faith judgment of the Corporation any or all of such conditions will not be satisfied. In addition, such notice will state that no representation is made as to the correctness or accuracy of the CUSIP, ISIN or similar number, if any, listed in such notice or printed on the Notes.

If any Note is to be redeemed in part only, the notice of redemption that relates to that Note will state the portion of the principal amount of that Note that is to be redeemed. A new Note in principal amount equal to the unredeemed portion of the original Note will be issued in the name of the holder of Notes upon cancellation of the original Note.

Payment of Additional Amounts

TransAlta will, subject to the exceptions and limitations set forth below, pay to any holder of Notes who is a non-resident of Canada under the Income Tax Act (Canada) (the “Tax Act”) such additional amounts as may be necessary so that the amount received by such holder on any payment made under or with respect to such Notes, after deduction or withholding by TransAlta or any of its paying agents for or on account of any present or future tax, duty, levy, assessment or other governmental charge (including penalties, interest and other liabilities related thereto) imposed or levied by or on behalf of the government of Canada or any province or territory thereof or by any authority or agency therein or thereof having power to tax (collectively, “Canadian Taxes”) upon or as a result of such payment, will not be less than the amount that the holder would have received if such Canadian Taxes had not been withheld or deducted. However, TransAlta will not be required to make any payment of additional amounts:

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nor will additional amounts be paid with respect to any payment on the Notes to a holder who is a fiduciary or partnership or other than the sole beneficial owner of such payment to the extent such payment would be required by the laws of Canada (or any political subdivision thereof) to be included in the income for Canadian federal income tax purposes of a beneficiary or settlor with respect to such fiduciary or a member of such partnership or a beneficial owner who would not have been entitled to payment of the additional amounts had such beneficiary, settlor, member or beneficial owner been the holder of the Notes.

The Corporation will furnish to the Trustee (for, upon request, forwarding to holders) of Notes, within 30 days after the date the payment of any Canadian Taxes is due pursuant to applicable law, certified copies of tax receipts or other documents evidencing such payment by the Corporation.

Wherever this Prospectus Supplement or in the Indenture there is mentioned, in any context, the payment of principal (and premium, if any), redemption price, interest or any other amount payable under or with respect to the Notes, such mention shall be deemed to include mention of the payment of additional amounts to the extent that, in such context, additional amounts are, were or would be payable in respect thereof (Section 10.5).

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For the purposes of Section 10.5 of the Indenture, a “Holder” of a Note shall include a beneficial owner thereof.

The foregoing is subject to certain further limitations, as described under the heading “Certain Income Tax Considerations – Certain U.S. Federal Income Tax Considerations – FATCA” in this Prospectus Supplement.

Events of Default

The following events are defined in the Indenture as “Events of Default” with respect to the Notes: (a) the failure of the Corporation to pay when due the principal of or premium (if any) on the Notes; (b) the failure of the Corporation, continuing for 30 days, to pay any interest due on the Notes; (c) the breach or violation of any covenant or condition (other than as referred to in (a) or (b) above), which continues for a period of 60 days after notice from the Trustee or from holders of at least 25% in principal amount of all outstanding Notes of any series affected thereby; (d) the failure of the Corporation or any Subsidiary to pay when due (after giving effect to any applicable grace periods) any amount owing in respect of any Indebtedness other than Non-Recourse Debt, or the Corporation or any Subsidiary otherwise defaults in the performance or observance of any other covenant, term, agreement or condition in connection with such Indebtedness, and if such Indebtedness has not matured it shall have been accelerated prior to the date on which the same would otherwise have become due and payable, provided that the aggregate principal amount of such Indebtedness is in excess of the greater of US$75 million and 3% of Consolidated Shareholders’ Equity (provided that if such default is waived by the persons entitled to do so, then the Event of Default in this clause (d) will be deemed to be waived without further action on the part of the Trustee or the holders of the Notes); (e) the entry of certain judgments or decrees against the Corporation or any Material Subsidiary for the payment of money in excess of the greater of US$75 million and 3% of Consolidated Shareholders’ Equity, in the aggregate, if the Corporation or any such Material Subsidiary, as the case may be, fails to pay such decree or judgment within 60 days or file an appeal thereof within 60 days or, if the Corporation or such Material Subsidiary, as the case may be does file an appeal, that judgment or decree continues undischarged or unstayed as provided in the Indenture; or (f) certain events of bankruptcy, insolvency or reorganization involving the Corporation or a Material Subsidiary.

If an Event of Default occurs and is continuing with respect to the Notes, then the Trustee or the holders of at least 25% in aggregate principal amount of the outstanding Notes may, declare the entire principal amount of all Notes and all interest thereon to be immediately due and payable. However, at any time after a declaration of acceleration with respect to the Notes has been made, but before a judgment or decree for payment of the money due has been obtained, the holders of a majority in principal amount of the outstanding Notes, by written notice to the Corporation and the Trustee, may under certain circumstances (which include payment or deposit with the Trustee of a sum sufficient to pay all unpaid principal and premium, if any, of, and all overdue interest, if any, on, those outstanding Notes and sums paid or advanced by the Trustee under the Indenture and the reasonable compensation, expenses, disbursements and advances of the Trustee, its agents and counsel), rescind and annul such declaration and its consequences (Section 5.2).

The Indenture provides that, subject to the duty of the Trustee during default to act with the required standard of care, the Trustee shall be under no obligation to exercise any of its rights and powers under the Indenture at the request or direction of any of the holders, unless such holders shall have offered to the Trustee reasonable indemnity (Section 6.2). Subject to such provisions for indemnification of the Trustee and certain other limitations set forth in the Indenture, the holders of a majority in principal amount of the outstanding debt securities of all series issued under the Indenture and affected by an Event of Default shall have the right to direct the time, method and place of conducting any proceeding for any remedy available to the Trustee, or exercising any trust or power conferred on the Trustee, with respect to the outstanding debt securities of all series issued under the Indenture and affected by such Event of Default (Section 5.12).

No holder of Notes will have any right to institute any proceeding with respect to the Indenture, or for the appointment of a receiver or a trustee, or for any other remedy thereunder, unless (a) such holder has previously

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given to the Trustee written notice of a continuing Event of Default with respect to the Notes, (b) the holders of at least 25% in aggregate principal amount of the outstanding Notes have made written request, and such holder or holders have offered reasonable indemnity, to the Trustee to institute such proceeding as Trustee, and (c) the Trustee has failed to institute such proceeding, and has not received from the holders of a majority in aggregate principal amount of the outstanding Notes a direction inconsistent with such request, within 60 days after such notice, request and offer (Section 5.7). However, such limitations do not apply to a suit instituted by the holder of Notes for the enforcement of payment of the principal of or any premium or interest on the Notes on or after the applicable due date therefor (Section 5.8).

The Corporation will be required to furnish to the Trustee annually a statement by certain of its officers as to whether or not the Corporation, to the best of their knowledge, is in compliance with all conditions and covenants of the Indenture and, if not, specifying all such known defaults (Section 10.4).

Mergers, Consolidations, Amalgamations and Sale of Assets

The Corporation will not enter into any transaction whereby all or substantially all of its undertaking, property and assets would become the property of any other person (the “Successor”), whether by reorganization, consolidation, amalgamation, arrangement, merger, transfer, sale, or otherwise, unless:

In addition to the above conditions, such transaction will substantially preserve and not impair any of the rights and powers of the Trustee or of the holders of Notes (Section 8.1).

If, as a result of any consolidation, amalgamation, arrangement, merger or upon any sale, conveyance, transfer or lease of all or substantially all of the properties and assets of the Corporation to any other person, any of the properties or assets of the Corporation or its Subsidiaries become subject to a Security Interest, then, unless such Security Interest could be created pursuant to the Indenture provisions described under “Negative Pledge” below without equally and rateably securing the Notes, the Corporation, simultaneously with or prior to such transaction, will cause the Notes to be secured equally and rateably with or prior to the Indebtedness secured by such Security Interest (Section 8.4).

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Covenants

The Indenture contains covenants substantially to the following effect:

Restriction on Sales and Leasebacks

The Corporation will not, and will not permit any Subsidiary to, enter into any sale and leaseback transaction unless the Corporation and its Subsidiaries comply with this restrictive covenant. A “sale and leaseback transaction” is an arrangement between the Corporation or any Subsidiary and a bank, insurance company or other lender or investor where the Corporation or any Subsidiary lease real or personal property which was or will be sold by the Corporation or any Subsidiary to that lender or investor. The Corporation can comply with this restrictive covenant if it meets either of the following conditions:

Negative Pledge

So long as any Notes remain outstanding the Corporation and its Subsidiaries will not create, assume or otherwise have outstanding any Security Interest, except for Permitted Encumbrances, on or over its or their respective assets (present or future) in respect of any Indebtedness of any person unless, in the opinion of legal counsel to the Corporation or the Trustee, the obligations of the Corporation in respect of all Notes then outstanding shall be secured equally and rateably therewith (Section 10.12).

Provision of Financial Information

TransAlta will file with the Trustee, within 15 days after it files them with the SEC, copies of its annual report and of the information, documents and other reports (or copies of such portions of any of the foregoing as the SEC may by rules and regulations prescribe) which TransAlta is required to file with the SEC pursuant to Section 13 or 15(d) of the U.S. Exchange Act. Notwithstanding that TransAlta may not be required to remain subject to the reporting requirements of Section 13 or 15(d) of the U.S. Exchange Act or otherwise report on an annual and quarterly basis on forms provided for such annual and quarterly reporting pursuant to rules and regulations promulgated by the SEC, TransAlta will continue to provide the Trustee (a) within 140 days after the end of each fiscal year, the information required to be contained in annual reports on Form 20-F or Form 40-F as applicable (or any successor form); and (b) within 60 days after the end of each of the first three fiscal quarters of each fiscal year, the information required to be contained in reports on Form 6-K (or any successor form), which, regardless of applicable requirements shall, at a minimum, consist of such information required to be provided in quarterly reports under the laws of Canada or any province thereof to security holders of a corporation with securities listed on the Toronto Stock Exchange, whether or not TransAlta has any of its securities listed on such exchange. Such information will be prepared in accordance with Canadian disclosure requirements and Generally Accepted Accounting Principles (Section 7.5).

Modification and Waiver

Modifications and amendments of the Indenture may be made by the Corporation and the Trustee with the consent of the holders of a majority in principal amount of the outstanding debt securities of each series issued under the Indenture affected by such modification or amendment; provided, however, that no such

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modification or amendment may, without the consent of the holder of each outstanding debt security of such affected series: (1) change the stated maturity of the principal of, or any instalment of interest, if any, on any debt security; (2) reduce the principal amount of, or the premium, if any, or the rate of interest, if any, on any debt security; (3) change the place of payment; (4) change the currency or currency unit of payment of principal of (or premium, if any) or interest, if any, on any debt security; (5) impair the right to institute suit for the enforcement of any payment on or with respect to any debt security; (6) adversely affect any right to convert or exchange any debt security; (7) reduce the percentage of principal amount of outstanding debt securities of such series, the consent of the holders of which is required for modification or amendment of the Indenture or for waiver of compliance with certain provisions of the Indenture or for waiver of certain defaults; (8) reduce the voting or quorum requirements relating to meetings of holders of debt securities; or (9) modify any provisions of the Indenture relating to the modification and amendment of the Indenture or the waiver of past defaults or covenants except as otherwise specified in the Indenture (Section 9.2).

The holders of a majority in principal amount of the outstanding Notes may on behalf of the holders of all Notes waive, insofar as the Notes are concerned, compliance by the Corporation with certain restrictive provisions of the Indenture (Section 10.13). The holders of a majority in principal amount of outstanding Notes may waive any past default under the Indenture with respect to the Notes, except a default in the payment of the principal of (or premium, if any) and interest, if any, on the Notes or in respect of a provision which under the Indenture cannot be modified or amended without the consent of the holder of each outstanding Note (Section 5.13). The Indenture or the Notes may be amended or supplemented, without the consent of any holder of Notes, to cure any ambiguity or inconsistency or to make any change that does not have an adverse effect on the rights of any holder of Notes (Section 9.1).

Defeasance

The Indenture provides that, at its option, TransAlta will be discharged from any and all obligations in respect of the Notes upon irrevocable deposit with the Trustee, in trust, of money, government securities or a combination thereof which will provide money in an amount sufficient in the opinion of a nationally recognized firm of independent chartered accountants to pay the principal of and premium, if any, and each instalment of interest, if any, on the outstanding Notes (“Defeasance”) (except with respect to the authentication, transfer, exchange or replacement of Notes or the maintenance of a place of payment and certain other obligations set forth in the Indenture). Such trust may only be established if among other things (1) TransAlta has delivered to the Trustee an opinion of counsel in the United States stating that (a) TransAlta has received from, or there has been published by, the Internal Revenue Service a ruling, or (b) since the date of execution of the Indenture, there has been a change in the applicable United States federal income tax law, in either case to the effect that the holders of such outstanding Notes will not recognize income, gain or loss for United States federal income tax purposes as a result of such Defeasance and will be subject to United States federal income tax on the same amounts, in the same manner and at the same times as would have been the case if such Defeasance had not occurred; (2) TransAlta has delivered to the Trustee an opinion of counsel in Canada or a ruling from the Canada Revenue Agency (“CRA”) to the effect that the holders of such outstanding Notes will not recognize income, gain or loss for Canadian federal, provincial or territorial income or other tax purposes as a result of such Defeasance and will be subject to Canadian federal or provincial income and other tax on the same amounts, in the same manner and at the same times as would have been the case had such Defeasance not occurred (and for the purposes of such opinion, such Canadian counsel shall assume that holders of the outstanding Notes include holders who are not resident in Canada); (3) no Event of Default or event that, with the passing of time or the giving of notice, or both, shall constitute an Event of Default shall have occurred and be continuing on the date of such deposit; (4) TransAlta is not an “insolvent person” within the meaning of the Bankruptcy and Insolvency Act (Canada); (5) TransAlta has delivered to the Trustee an opinion of counsel to the effect that such deposit shall not cause the Trustee or the trust so created to be subject to the United States Investment Company Act of 1940, as amended; and (6) TransAlta has delivered to the Trustee an officer’s certificate and opinion of counsel stating that certain conditions precedent are satisfied. TransAlta may exercise its Defeasance option notwithstanding its prior exercise of its Covenant

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Defeasance option described in the following paragraph if TransAlta meets the conditions described in the preceding sentence at the time TransAlta exercises the Defeasance option (Sections 14.1, 14.2 and 14.4).

The Indenture provides that, at its option, unless and until TransAlta has exercised its Defeasance option described in the preceding paragraph, TransAlta may omit to comply with covenants, including the covenants described above under the heading “Covenants”, and such omission shall not be deemed to be an Event of Default under the Indenture and the outstanding Notes upon irrevocable deposit with the Trustee, in trust, of money and/or government securities which will provide money in an amount sufficient in the opinion of a nationally recognized firm of independent chartered accountants to pay the principal of and premium, if any, and each instalment of interest, if any, on the outstanding Notes (“Covenant Defeasance”). If TransAlta exercises its Covenant Defeasance option, the obligations under the Indenture other than with respect to such covenants and the Events of Default other than with respect to such covenants shall remain in full force and effect. Such trust may only be established if, among other things, (1) TransAlta has delivered to the Trustee an opinion of counsel in the United States to the effect that the holders of the outstanding Notes will not recognize income, gain or loss for United States federal income tax purposes as a result of such Covenant Defeasance and will be subject to United States federal income tax on the same amounts, in the same manner and at the same times as would have been the case if such Covenant Defeasance had not occurred; (2) TransAlta has delivered to the Trustee an opinion of counsel in Canada or a ruling from the CRA to the effect that the holders of such outstanding Notes will not recognize income, gain or loss for Canadian federal, provincial or territorial income or other tax purposes as a result of such Covenant Defeasance and will be subject to Canadian federal or provincial income and other tax on the same amounts, in the same manner and at the same times as would have been the case had such Covenant Defeasance not occurred (and for the purposes of such opinion, such Canadian counsel shall assume that holders of the outstanding Notes include holders who are not resident in Canada); (3) no Event of Default or event that, with the passing of time or the giving of notice, or both, shall constitute an Event of Default shall have occurred and be continuing on the date of such deposit; (4) TransAlta is not an “insolvent person” within the meaning of the Bankruptcy and Insolvency Act (Canada); (5) TransAlta has delivered to the Trustee an opinion of counsel to the effect that such deposit shall not cause the Trustee or the trust so created to be subject to the United States Investment Company Act of 1940, as amended; and (6) TransAlta has delivered to the Trustee an officer’s certificate and opinion of counsel stating that certain conditions precedent are satisfied (Sections 14.3 and 14.4).

Governing Law

The Notes and the Indenture will be governed by and construed in accordance with the laws of the State of New York (Section 1.11).

Definitions

The Indenture contains, among others, definitions substantially to the following effect:

“Attributable Amount” means with respect to any sale and leaseback transaction (as defined herein under the heading “Covenants – Restrictions on Sales and Leasebacks” below), as at the time of determination, the present value (discounted at the rate of interest set forth or implicit in the terms of such lease, compounded annually) of the total obligations of the lessee for rental payments during the remaining term of the lease included in such sale and leaseback transaction.

“Consolidated Net Tangible Assets” means all consolidated assets of the Corporation as shown on the most recent audited consolidated balance sheet of the Corporation, less the aggregate of the following amounts reflected upon such balance sheet: (i) all goodwill, deferred assets, trademarks, copyrights and other similar intangible assets; (ii) to the extent not already deducted in computing such assets and without duplication, depreciation, depletion, amortization, reserves and any other account which reflects a decrease in the value of an asset or a periodic allocation of the cost of an asset; provided that no such deduction shall be made to the extent

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such account reflects a decrease in the value or periodic allocation of the cost of any assets referred to in (i) above; (iii) minority interests; (iv) current liabilities; and (v) assets created, developed, constructed or acquired with or in respect of which Non-Recourse Debt has been incurred, and any and all receivables, inventory, equipment, chattel paper, intangibles and other rights or collateral arising from or connected with those assets (including the shares or other ownership interests of a single purpose entity which holds only such assets and other rights and collateral arising from or connected therewith) and to which recourse of the lender of such Non-Recourse Debt is limited to the extent of the outstanding Non-Recourse Debt financing such assets.

“Consolidated Shareholders’ Equity” means, without duplication, the aggregate amount of shareholders’ equity (including, without limitation, common share capital, preferred share capital, contributed surplus and retained earnings) of the Corporation as shown on the most recent audited consolidated balance sheet of the Corporation, adjusted by the amount by which common share capital, preferred share capital and contributed surplus has been increased or decreased (as the case may be) from the date of such balance sheet to the relevant date of determination, in accordance with Generally Accepted Accounting Principles, together with the aggregate principal amount of obligations of the Corporation in respect of Preferred Securities.

“Financial Instrument Obligations” means obligations arising under:

to the extent of the net amount due or accruing due by the Corporation thereunder (determined by marking to market the same in accordance with their terms).

“Generally Accepted Accounting Principles” means generally accepted accounting principles which are in effect from time to time in Canada.

“Indebtedness” means all items of indebtedness in respect of any amounts borrowed (including obligations with respect to bankers’ acceptances and contingent reimbursement obligations relating to letters of credit and other financial instruments) and all Purchase Money Obligations which, in accordance with Generally Accepted Accounting Principles, would be recorded in the financial statements as at the date as of which Indebtedness is to be determined, and in any event including, without duplication: (i) obligations secured by any Security Interest existing on property owned subject to such Security Interest, whether or not the obligations secured thereby shall have been assumed; and (ii) guarantees, indemnities, endorsements (other than

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endorsements for collection in the ordinary course of business) or other contingent liabilities in respect of obligations of another person for indebtedness of that other person in respect of any amounts borrowed by them.

“Material Subsidiary” means, at any time, a Subsidiary (i) the total assets of which represent more than 10% of the total assets of the Corporation determined on a consolidated basis as shown in the most recent audited consolidated balance sheet of the Corporation; or (ii) the total revenues of which represent more than 10% of the total revenues of the Corporation determined on a consolidated basis as shown in the consolidated income statement of the Corporation for the four most recent fiscal quarters of the Corporation.

“Non-Recourse Debt” means any Indebtedness incurred to finance the creation, development, construction or acquisition of assets and any increases in or extensions, renewals or refundings of any such Indebtedness, provided that the recourse of the lender thereof or any agent, trustee, receiver or other person acting on behalf of the lender in respect of such Indebtedness in respect thereof is limited in all circumstances (other than in respect of false or misleading representations or warranties and customary indemnities provided with respect to such financings) to the assets created, developed, constructed or acquired in respect of which such Indebtedness has been incurred and to any receivables, inventory, equipment, chattel paper, intangibles and other rights or collateral arising from or connected with the assets so created, developed, constructed or acquired (including the shares or other ownership interests of a single purpose entity which holds only such assets and other rights and collateral arising from or connected therewith) and to which the lender has recourse.

“Permitted Encumbrance” means any of the following:

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“Preferred Securities” means securities which on the date of issue thereof by a Person: (i) have a term to maturity of more than 30 years; (ii) rank subordinate to the unsecured and unsubordinated Indebtedness of such person outstanding on such date; (iii) entitle such person to defer the payment of interest thereon for more than four years without thereby causing an event of default in respect of such securities to occur; and (iv) entitle such person to satisfy the obligation to make payments of deferred interest thereon from the proceeds of the issuance of its shares.

“Purchase Money Obligation” means any monetary obligation created or assumed as part of the purchase price of real or tangible personal property, whether or not secured, any extensions, renewals, alterations or replacements of any such obligation, provided that the principal amount of such obligation outstanding on the date of such extension, renewal, alteration or replacement is not increased and further provided that any security given in respect of such obligation shall not extend to any property other than the property acquired in connection with which such obligation was created or assumed and fixed improvements, if any, erected or constructed thereon.

“Security Interest” means any mortgage, charge, pledge, lien, encumbrance, assignment by way of security, title retention agreement or other security interest whatsoever, howsoever created or arising, whether absolute or contingent, fixed or floating, perfected or not, which secures payment or performance of an obligation.

“Subsidiary” means, in relation to a Person (a) any corporation of which at least a majority of the outstanding shares having by the terms thereof ordinary voting power to elect a majority of the board of directors of such corporation (irrespective of whether at the time shares of any other class or classes of such corporation might have voting power by reason of the happening of any contingency, unless the contingency has occurred and then only for as long as it continues) is at the time directly, indirectly or beneficially owned or controlled by the person or one or more of its Subsidiaries, or the person and one or more of its Subsidiaries; (b) any partnership of which the person or one or more of its Subsidiaries, or the person and one or more of its Subsidiaries: (i) directly, indirectly or beneficially own or control more than 50% of the income, capital, beneficial or ownership interests (however designated) thereof; and (ii) is a general partner, in the case of a limited partnership, or is a partner that has authority to bind the partnership, in all other cases; or (iii) any other person of which at least a majority of the income, capital, beneficial or ownership interests (however designated) are at the time directly, indirectly or beneficially owned or controlled by the first mentioned person or one or more of its Subsidiaries, or the first mentioned person and one or more of its Subsidiaries.

“Wholly-Owned Subsidiary” means any Subsidiary that the Corporation directly or indirectly beneficially owns 100% of the outstanding shares having by the terms thereof ordinary voting power to elect a majority of the board of directors of such Subsidiary or owns, directly or indirectly, 100% of the income, capital, beneficial or ownership interests (however designated) thereof.

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Book-Entry System

DTC is a limited-purpose trust company organized under the New York Banking Law, a “banking organization” within the meaning of the New York Banking Law, a member of the Federal Reserve System, a “clearing corporation” within the meaning of the New York Uniform Commercial Code, and a “clearing agency” registered pursuant to the provisions of Section 17A of the U.S. Exchange Act. DTC holds and provides asset servicing for issues of U.S. and non-U.S. equity issues, corporate and municipal debt issues, and money market instruments that DTC’s participants (“Direct Participants”) deposit with DTC. DTC also facilitates the post-trade settlement among Direct Participants of sales and other securities transactions in deposited securities, through electronic computerized book-entry transfers and pledges between Direct Participants’ accounts. This eliminates the need for physical movement of securities certificates. Direct Participants include both U.S. and non-U.S. securities brokers and dealers, banks, trust companies, clearing corporations, and certain other organizations. DTC is a wholly-owned subsidiary of The Depository Trust & Clearing Corporation (“DTCC”). DTCC is the holding company for DTC, National Securities Clearing Corporation and Fixed Income Clearing Corporation, all of which are registered clearing agencies. DTCC is owned by the users of its regulated subsidiaries. Access to the DTC system is also available to others such as both U.S. and non-U.S. securities brokers and dealers, banks, trust companies, and clearing corporations that clear through or maintain a custodial relationship with a Direct Participant, either directly or indirectly (“Indirect Participants”). The DTC Rules applicable to its Participants are on file with the SEC. More information about DTC can be found at www.dtcc.com and www.dtc.org.

So long as DTC or a nominee thereof is the registered owner of the global security representing the Notes (the “Global Security”), DTC or such nominee, as the case may be, will be considered the sole owner or holder of Notes represented thereby for all purposes under the Indenture. Except as provided below, owners of beneficial interests in Notes represented by a Global Security (“Beneficial Owners”) will not be entitled to have such Notes registered in their names, will not receive or be entitled to receive physical delivery of such Notes in definitive form and will not be considered the owners or holders thereof under the Indenture.

Unless and until it is exchanged in whole or in part for Notes in definitive form, no Global Security may be transferred except as a whole by DTC to a nominee of DTC or by a nominee of DTC to DTC or another nominee of DTC or by DTC or any such nominee to a successor of DTC or a nominee of such successor.

Purchases of Notes under the DTC system must be made by or through Direct Participants, which will receive a credit for the Notes on DTC’s records. The ownership interest of each Beneficial Owner is in turn to be recorded on the Direct and Indirect Participants’ records. Beneficial Owners will not receive written confirmation from DTC of their purchase. Beneficial Owners are, however, expected to receive written confirmations providing details of the transaction, as well as periodic statements of their holdings, from the Direct or Indirect Participant through which the Beneficial Owner entered into the transaction. Transfers of ownership interests in the Notes are to be accomplished by entries made on the books of Direct and Indirect Participants acting on behalf of Beneficial Owners. Beneficial Owners will not receive certificates representing their ownership interests in the Notes, except in the event that use of the book-entry system for the Notes is discontinued.

To facilitate subsequent transfers, all Notes deposited by Direct Participants with DTC are registered in the name of DTC’s partnership nominee, Cede & Co., or such other name as may be requested by an authorized representative of DTC. The deposit of Notes with DTC and their registration in the name of Cede & Co. or such other DTC nominee do not affect any change in beneficial ownership. DTC has no knowledge of the actual Beneficial Owners of the Notes; DTC’s records reflect only the identity of the Direct Participants to whose accounts such Notes are credited, which may or may not be the Beneficial Owners. The Direct and Indirect Participants will remain responsible for keeping account of their holdings on behalf of their customers.

Conveyance of notices and other communications by DTC to Direct Participants, by Direct Participants to Indirect Participants, and by Direct Participants and Indirect Participants to Beneficial Owners will be governed

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by arrangements among them, subject to any statutory or regulatory requirements as may be in effect from time to time. Beneficial Owners of Notes may wish to take certain steps to augment the transmission to them of notices of significant events with respect to the Notes, such as redemptions, tenders, defaults, and proposed amendments to the Notes. For example, Beneficial Owners of Notes may wish to ascertain that the nominee holding the Notes for their benefit has agreed to obtain and transmit notices to Beneficial Owners. In the alternative, Beneficial Owners may wish to provide their names and addresses to the registrar and request that copies of notices be provided directly to them.

Neither DTC nor Cede & Co. (nor any other DTC nominee) will consent or vote with respect to Notes unless authorized by a Direct Participant in accordance with DTC’s Money Market Instrument procedures. Under its usual procedures, DTC mails an “Omnibus Proxy” to the Corporation as soon as possible after the record date. The Omnibus Proxy assigns Cede & Co.’s consenting or voting rights to those Direct Participants to whose accounts Notes are credited on the record date (identified in a listing attached to the Omnibus Proxy).

Payments on the Notes will be made to Cede & Co., or such other nominee as may be requested by an authorized representative of DTC. DTC’s practice is to credit Direct Participants’ accounts upon DTC’s receipt of funds and corresponding detail information from the Corporation or the Trustee, on payable date in accordance with their respective holdings shown on DTC’s records. Payments by Participants to Beneficial Owners will be governed by standing instructions and customary practices, as is the case with securities held for the accounts of customers in bearer form or registered in “street name”, and will be the responsibility of such Participant and not of DTC, the Trustee, or the Corporation, subject to any statutory or regulatory requirements as may be in effect from time to time. Any payment due to Cede & Co. (or such other nominee as may be requested by an authorized representative of DTC) is the Corporation’s responsibility or the responsibility of the Trustee, disbursement of such payments to Direct Participants shall be the responsibility of DTC, and disbursement of such payments to the Beneficial Owners shall be the responsibility of Direct Participants and Indirect Participants.

None of TransAlta, the Trustee or any paying agent for the Notes will have any responsibility or liability for any aspect of the records relating to, or payments made on account of, beneficial ownership interests of the Notes or for maintaining, supervising or reviewing any records relating to such beneficial ownership interests.

DTC may discontinue providing its services as depository with respect to the Notes at any time by giving reasonable notice to the Corporation or the Trustee. Under such circumstances, in the event that a successor depository is not obtained, certificates for the Notes are required to be printed and delivered.

The Corporation may decide to discontinue use of the system of book-entry-only transfers through DTC (or a successor Notes depository). In that event, certificates will be printed and delivered to DTC.

The information in this section covering DTC and DTC’s system has been obtained from sources that the Corporation believes to be reliable, but the Corporation takes no responsibility for the accuracy thereof. The information in this section is subject to any changes to the arrangements between the Corporation and DTC and any changes to these procedures that may be instituted unilaterally by DTC.

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EARNINGS COVERAGE

The following earnings coverage ratios have been prepared in accordance with Canadian securities law requirements and are included in this Prospectus Supplement in accordance with Canadian disclosure requirements.

The following table sets forth our earnings coverage ratios calculated for the twelve-month period ended December 31, 2024 and the twelve-month period ended September 30, 2025 after giving pro forma effect to the issuance of the Notes, as described in this Prospectus Supplement, and the intended use of proceeds therefrom. Adjustments for normal course issuances and repayments of long-term debt subsequent to December 31, 2024 would not materially affect the ratios set forth below and have not been made.

The earnings coverage ratios set forth above do not purport to be indicative of earnings coverage ratios for any future periods. The earnings coverage ratios have been calculated based on information prepared in accordance with IFRS.

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CERTAIN INCOME TAX CONSIDERATIONS

The following summary is of a general nature only and is not intended to be, and should not be construed to be, legal or tax advice to any prospective investor and no representation with respect to the tax consequences to any particular investor is made. Accordingly, prospective investors should consult with their own tax advisors for advice with respect to the income tax consequences to them having regard to their own particular circumstances, including any consequences of an investment in the Notes arising under state, provincial or local tax laws in the United States or Canada or tax laws of jurisdictions outside the United States or Canada.

Certain Canadian Federal Income Tax Considerations

The following summary, as of the date hereof, describes the principal Canadian federal income tax considerations generally applicable to a purchaser of Notes pursuant to this Prospectus Supplement and the Prospectus who, at all relevant times, for purposes of the Tax Act and any applicable tax treaty: (i) is neither resident nor deemed to be resident in Canada; (ii) acquires the Notes as beneficial owner under this Prospectus Supplement; (iii) deals at arm’s length with, and is not affiliated with, us and with any transferee who is resident in Canada (or deemed to be resident in Canada) for purposes of the Tax Act and to whom the purchaser assigns or otherwise transfers a Note; (iv) does not use or hold and is not deemed to use or hold a Note in carrying on business in Canada; (v) is not an insurer who carries on an insurance business, or is deemed to carry on an insurance business, in Canada and elsewhere; (vi) is not an “authorized foreign bank” as defined in the Tax Act; (vii) is entitled, as beneficial owner, to receive all payments (including principal, interest and premium, if any) in respect of a Note; and (viii) is not a “specified non-resident shareholder” of us within the meaning of subsection 18(5) of the Tax Act or a person that does not deal at arm’s length with a “specified shareholder” within the meaning of subsection 18(5) (each such purchaser is referred to herein as a “Non-Resident Holder”).

This summary is based on the current provisions of the Tax Act, the regulations thereunder (the “Regulations”) and counsel’s understanding of the current administrative policies and assessing practices of the CRA published in writing, publicly available and in effect as of the date hereof. This summary also takes into account all specific proposals to amend the Tax Act and the Regulations publicly announced by or on behalf of the Minister of Finance (Canada) prior to the date of this Prospectus Supplement (the “Proposed Amendments”) and assumes that all Proposed Amendments will be enacted in the form proposed. No assurance can be given that the Proposed Amendments will be enacted in the form proposed, or at all. Other than the Proposed Amendments, this summary does not take into account or anticipate any changes in law or CRA administrative policies or assessing practices, whether by legislative, governmental or judicial action or interpretation, nor does it take into account provincial, territorial or foreign tax considerations, which may differ significantly from those discussed herein.

This summary does not address the possible application of the “hybrid mismatch arrangement” rules contained in section 18.4 of the Tax Act to a Non-Resident Holder (i) that disposes of a Note to a person or entity with which it does not deal at arm’s length or to an entity that is a “specified entity” (as defined in subsection 18.4(1) of the Tax Act) with respect to the Non-Resident Holder or in respect of which the Non-Resident Holder is a “specified entity”, (ii) that disposes of a Note under, or in connection with, a “structured arrangement” (as defined in subsection 18.4(1) of the Tax Act), or (iii) in respect of which we are a “specified entity”. Such Non-Resident Holders should consult their own tax advisors.

This summary is of a general nature only and is not intended to be, nor should it be construed to be, legal or tax advice to any particular purchaser and no representations with respect to the income tax consequences to any particular purchaser are made. This summary is not exhaustive of all Canadian federal income tax considerations. Accordingly, prospective purchasers of Notes should consult their own tax advisors with respect to their own particular circumstances.

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Under the Tax Act, interest, principal, discount or premium, if any, paid or credited, or deemed to be paid or credited, by the Company to a Non-Resident Holder on the Notes, and any proceeds of disposition received by a Non-Resident Holder on the disposition of a Note including on redemption, payment on maturity or repurchase, will be exempt from Canadian withholding tax. No other taxes on income (including taxable capital gains) will be payable under the Tax Act by a Non-Resident Holder on interest, principal, discount or premium, or on the proceeds received by a Non-Resident Holder on the disposition of a Note including on redemption, payment on maturity or repurchase, solely as a consequence of the acquisition, holding or disposition (including on redemption, payment on maturity or repurchase) of the Notes.

Certain U.S. Federal Income Tax Considerations

The following is a summary of the certain U.S. federal income tax considerations relevant to the acquisition, ownership and disposition of a Note by an initial purchaser thereof who is a U.S. Holder (as hereinafter defined) who purchases the Note for cash at its “issue price” (the first price at which a substantial amount of the Notes is sold for cash, excluding sales to bond houses, brokers, or similar persons acting in the capacity of underwriters, placement agents or wholesalers) and who will hold the Note as a “capital asset” within the meaning of Section 1221 of the Internal Revenue Code of 1986, as amended (the “Code”). This summary is intended for general information only and does not address all potentially relevant U.S. federal income tax matters.

This summary does not address the tax consequences to U.S. Holders subject to special provisions of the Code including, without limitation: banks and other financial institutions; thrifts; tax-exempt entities or organizations; insurance companies; regulated investment companies or real estate investment trusts; holders subject to the alternative minimum tax; certain former citizens or residents of the United States; dealers in securities or foreign currencies that elect to use a mark-to-market method of accounting; traders that mark-to-market their securities; qualified retirement plans; individual retirement accounts or other tax-deferred accounts; holders holding Notes as part of a “hedge”, “straddle”, “conversion transaction,” “wash sale” or other integrated transaction; entities or arrangements that are treated as partnerships or other pass-through entities for U.S. federal income tax purposes (and investors therein); persons required for U.S. federal income tax purposes to conform the timing of income accruals with respect to their Notes to their financial statements under Section 451 of the Code; persons holding Notes that are attributable to an office or other fixed place of business maintained outside of the United States; persons holding 2029 Notes whose 2029 Notes are redeemed; and holders with a “functional currency” other than the U.S. dollar. This summary also does not cover any state, local or non-U.S. tax consequences. This summary is based upon existing provisions of the Code, final and temporary Treasury regulations promulgated thereunder (“U.S. Treasury Regulations”), and rulings and judicial decisions in effect on the date hereof, all of which are subject to change or differing interpretation (possibly with retroactive effect), so as to result in U.S. federal income tax consequences different from those described herein. This discussion is not binding on the U.S. Internal Revenue Service (the “IRS”) and we have not sought and will not seek any ruling from the IRS regarding the matters discussed below. There can be no assurance that the IRS will not take positions that are different from those discussed below or that a U.S. court will not sustain such a challenge.

As used herein, the term “U.S. Holder” means a beneficial owner of a Note that, for U.S. federal income tax purposes, is (i) an individual who is a citizen or resident of the United States, (ii) a corporation created or organized under the laws of the United States, any state thereof or the District of Columbia, (iii) an estate the income of which is subject to U.S. federal income tax without regard to its source, or (iv) a trust if a court within the United States is able to exercise primary supervision over the administration of the trust and one or more United States persons have the authority to control all substantial decisions of the trust or if the trust has a valid election in effect to be treated as a United States person.

If an entity or arrangement treated as a partnership for U.S. federal income tax purposes holds Notes, the U.S. federal income tax treatment of a partner in the partnership will depend on the status of the partner and the

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activities of the partnership. Entities or arrangements treated as partnerships for U.S. federal income tax purposes that are considering an investment in Notes and investors therein should consult their tax advisors regarding the tax consequences of the acquisition, ownership or disposition of the Notes.

Effect of Certain Contingencies

In certain circumstances we may be obligated to pay amounts in excess of stated interest or principal on the Notes (see, for example, “Description of Debt Securities – Payment of Additional Amounts” in the Prospectus and “Description of Notes–Optional Redemption,” “Description of Notes – Repurchase Upon Change of Control Triggering Event” and “Description of Notes – Tax Redemption” in this Prospectus Supplement). These potential payments may implicate the U.S. Treasury Regulations relating to “contingent payment debt instruments.” However, under the applicable U.S. Treasury Regulations, such contingent payments should not cause the Notes to be contingent payment debt instruments if, based on all the facts and circumstances as of the date on which the Notes are issued, there is only a remote likelihood that any contingencies causing the payment of such excess amounts will occur, if such excess amounts, in the aggregate, are considered incidental or if another exception applies. We believe and, to the extent required to take a position, intend to take the position that the possibility of paying excess amounts should not cause the Notes to be contingent payment debt instruments. Our position will be binding on a U.S. Holder unless such holder timely and explicitly discloses its contrary position in the manner required by applicable U.S. Treasury Regulations. Our position, however, is not binding on the IRS. If the IRS successfully challenges this position, a U.S. Holder may be required to accrue interest income on its Notes at a rate in excess of the stated interest rate and to treat any gain recognized on the taxable disposition of a Note as ordinary income rather than as capital gain. Prospective U.S. Holders should consult their own tax advisors regarding this issue. The remainder of this discussion assumes that the Notes will not be treated as contingent payment debt instruments.

Interest

Interest (including, for purposes of this discussion, any taxes withheld therefrom and any additional amounts paid with respect thereto) on the Notes will generally be taxable to a U.S. Holder as ordinary income at the time it is received or accrued in accordance with such holder’s regular method of accounting for U.S. federal income tax purposes. Such interest generally will constitute income from sources outside the United States and passive category income for U.S. foreign tax credit limitation purposes. The rules governing U.S. foreign tax credits are complex. Prospective purchasers of Notes should consult their tax advisors regarding the availability of U.S. foreign tax credits in their particular circumstances.

Sale, Exchange, Retirement, Redemption or Other Taxable Disposition of a Note

Upon the sale, exchange, retirement, redemption or other taxable disposition of a Note, a U.S. Holder generally will recognize gain or loss equal to the difference, if any, between the amount realized on such disposition (other than amounts received that are attributable to accrued but unpaid interest, which amounts will be taxable as ordinary income to the extent not previously included in income) and such U.S. Holder’s adjusted tax basis in the Note, which generally is its cost. Such gain or loss generally will constitute capital gain or loss and generally will be long-term capital gain or loss if the Note was held by such U.S. Holder for more than one year. Net long-term capital gain of non-corporate U.S. Holders, including individuals, is generally eligible for reduced rates of taxation. The deductibility of capital losses is subject to limitations.

Gain or loss recognized by a U.S. Holder generally will be treated as U.S. source income or loss for foreign tax credit purposes. Prospective investors should consult their own tax advisors as to the foreign tax credit implications of such taxable disposition of a Note.

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Additional Tax on Investment Income

Certain U.S. Holders who are individuals, estates or trusts and whose income exceeds certain thresholds generally will be required to pay an additional 3.8 percent tax on all or a portion of their “net investment income,” which includes, among other things, interest income and capital gains from the sale or other taxable disposition (including retirement or redemption) of a Note, subject to certain limitations and exceptions. Prospective U.S. Holders should consult their own tax advisors regarding the application of this additional tax to their investment in the Notes.

Information Reporting and Backup Withholding

In general, payments of interest and principal on and the proceeds from sales of Notes held by a U.S. Holder may be required to be reported to the IRS unless the U.S. Holder is a corporation or other exempt recipient and, when required, demonstrates this fact. In addition, a U.S. Holder that is not an exempt recipient may be subject to backup withholding of U.S. federal income tax on such payments unless it provides its taxpayer identification number and otherwise complies with applicable certification requirements. Backup withholding is not an additional tax. The amount of any backup withholding from a payment to a U.S. Holder generally will be allowed as a credit against such U.S. Holder’s U.S. federal income tax liability and may entitle such U.S. Holder to a refund, provided that the required information is furnished to the IRS in a timely manner. Prospective U.S. Holders should consult their tax advisors regarding the application of backup withholding, the availability of an exemption from backup withholding and the procedure for obtaining such an exemption, if available.

Information with Respect to Foreign Financial Assets

Certain U.S. Holders (including individuals) who hold an interest in “specified foreign financial assets” (as defined in Section 6038D of the Code) with an aggregate value exceeding certain threshold amounts generally are required to report information relating to such assets with their tax returns (generally, on IRS Form 8938), subject to certain exceptions. The Notes generally will constitute specified foreign financial assets subject to these reporting requirements unless the Notes are held in accounts maintained by certain financial institutions (in which case the accounts themselves may be reportable if maintained by non-U.S. financial institutions).

Prospective U.S. Holders should consult their own tax advisors regarding application of the foregoing disclosure requirements to their ownership of the Notes and the potentially significant penalties for, and other adverse consequences of, non-compliance.

FATCA

No additional amounts will be required to be paid on account of, and payments on the Notes will be paid net of, any deduction or withholding imposed under Sections 1471 through 1474 of the Code (provisions commonly known as “FATCA”) or any current or future regulations issued thereunder, any intergovernmental agreement entered into with respect to FATCA or similar law or regulation adopted pursuant to an intergovernmental agreement between a non-U.S. jurisdiction and the United States with respect to the foregoing or any agreements entered into pursuant to Section 1471(b)(1) of the Code.

THE U.S. FEDERAL INCOME TAX DISCUSSION SET FORTH ABOVE IS INCLUDED FOR GENERAL INFORMATION ONLY AND MAY NOT BE APPLICABLE DEPENDING UPON A PROSPECTIVE PURCHASER’S PARTICULAR SITUATION. PROSPECTIVE PURCHASERS ARE URGED TO CONSULT THEIR OWN TAX ADVISORS WITH RESPECT TO THE TAX CONSEQUENCES TO THEM OF THE ACQUISITION, OWNERSHIP AND DISPOSITION OF THE NOTES INCLUDING THE TAX CONSEQUENCES UNDER U.S. FEDERAL, STATE AND LOCAL, NON-U.S. AND OTHER TAX LAWS AND THE POSSIBLE EFFECTS OF CHANGES IN U.S. OR OTHER TAX LAWS.

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UNDERWRITING

We intend to offer the Notes through the underwriters named below for whom RBC Capital Markets, LLC (the “Representative”) is acting as representative. Subject to the terms and conditions contained in the underwriting agreement dated the date of this Prospectus Supplement, each underwriter has severally agreed to purchase, and we have agreed to sell to such underwriter, the principal amount of Notes set forth opposite the underwriter’s name.

In the underwriting agreement, the underwriters have severally agreed, subject to the terms and conditions set forth therein, to purchase all the Notes offered under this Prospectus Supplement if any of the Notes are purchased. In the event of default by an underwriter, the underwriting agreement provides that, in certain circumstances, purchase commitments of the non-defaulting underwriters may be increased or the underwriting agreement may be terminated. The obligations of the underwriters under the underwriting agreement may also be terminated upon the occurrence of certain stated events specified in the underwriting agreement including “regulatory out”, “litigation out”, “disaster out” and “material change out” provisions.

The underwriting agreement provides that the obligations of the underwriters to purchase the Notes are subject to approval of legal matters by counsel and to other conditions.

The underwriters propose to offer the Notes directly to the public at the public offering price set forth on the cover page of this Prospectus Supplement and to dealers at the public offering price, less a concession not to exceed   % of the principal amount of the Notes. The underwriters may allow, and dealers may re-allow a concession not to exceed    % of the principal amount of the Notes on sales to other dealers. After the underwriters have made a reasonable effort to sell all of the Notes offered by this Prospectus Supplement at the price set forth on the cover page of this Prospectus Supplement, the offering price of the Notes may be decreased and may be further changed from time to time to an amount not greater than the price set forth on the cover page of this Prospectus Supplement. Any such reduction will not affect the proceeds we receive pursuant to the offering.

The underwriting agreement provides that, in consideration of the services of the underwriters in connection with the offering of Notes, we will pay the underwriters a commission equal to   % of the

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principal amount of Notes sold pursuant to the offering, for an aggregate commission payable by us of U.S.$   . The underwriters’ commission is payable on the closing of the offering.

The terms of the offering were established through negotiations between us and the underwriters.

We have agreed to indemnify the underwriters against certain liabilities, including liabilities under the U.S. Securities Act, or to contribute to payments the underwriters may be required to make because of any such liabilities.

In connection with this offering, the Representative may purchase and sell Notes in the open market. These transactions may include over-allotment, syndicate covering transactions and stabilizing transactions. Over-allotment involves syndicate sales of the Notes in excess of the principal amount of the Notes to be purchased by the underwriters in this offering, which creates a syndicate short position. Syndicate covering transactions involve purchases of the Notes in the open market after the distribution has been completed in order to cover syndicate short positions. Stabilizing transactions consist of certain bids or purchases of the Notes made for the purpose of preventing or retarding a decline in the market price of the Notes while this offering is in progress. The underwriters may also impose a penalty bid. Penalty bids permit the underwriters to reclaim a selling concession from a syndicate member when the Representative, in covering syndicate short positions or making stabilizing purchases, repurchases Notes originally sold by that syndicate member.

As a result of these activities, the market price of the Notes offered under this Prospectus Supplement may be higher than the price that otherwise might exist in the open market. If these activities are commenced, they may be discontinued by the Representative at any time without notice. The Representative may carry out these transactions in the over-the-counter market or otherwise. Neither we nor any of the underwriters make any representation or prediction as to the direction or magnitude of any effect that the transactions described above may have on the price of the Notes.

We estimate that our total expenses for this offering will be approximately US$    (not including underwriting commissions).

Relationship Between TransAlta and the Underwriters

Certain of the underwriters are, directly or indirectly, subsidiaries or affiliates of lenders (collectively, the “Lenders”) that have extended credit facilities (collectively, the “Facilities”) to us or our subsidiaries and to which we or our subsidiaries are currently indebted. In addition, certain of the underwriters, or their subsidiaries or affiliates, may be holders of the 2029 Notes. As described in “Use of Proceeds”, we intend to use the net proceeds from this offering, together with cash on hand, to redeem the 2029 Notes. To the extent that the underwriters (or their respective subsidiaries or affiliates) are holders of 2029 Notes, they may receive a portion of the net proceeds from this offering of Notes. Accordingly, we may be considered to be a “connected issuer” of such underwriters under applicable Canadian securities legislation. As of December 10, 2025, we and our subsidiaries were indebted to the Lenders under the Facilities in the aggregate amount of approximately $204 million. As of the date hereof, we and our subsidiaries are in compliance with all material terms of the agreements governing the Facilities and none of the Lenders has waived any breach by us or our subsidiaries of those agreements since the Facilities were established. Our financial position has not changed substantially and adversely since the indebtedness under the Facilities was incurred.

RBC Capital Markets, LLC, together with its affiliates, RBC Global Asset Management Inc., RBC Phillips Hager & North Investment Counsel Inc., RBC Private Counsel (USA) Inc., Royal Trust Corporation of Canada, GFC Partnership and RT Partnership, collectively own approximately 9.5% of the issued and outstanding common shares of the Corporation. Accordingly, the Corporation may be considered a “related issuer” or a “connected issuer” to RBC Capital Markets, LLC for purposes of applicable Canadian securities laws.

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Certain of the underwriters and their respective affiliates have in the past performed, and may in the future perform, various financial advisory, investment banking and commercial lending service for us and our affiliates in the ordinary course of business, for which they have received and will receive customary fees and commissions.

In the ordinary course of their business activities, the underwriters and their affiliates may make or hold a broad array of investments and actively trade debt and equity securities (or related derivative securities) and financial instruments (including bank loans) for their own account and for the accounts of their customers. Such investments and securities activities may involve securities and/or instruments of ours or our affiliates. Certain of the underwriters or their affiliates that have a lending relationship with us routinely hedge their credit exposure to us consistent with their customary risk management policies. Typically, such underwriters and their affiliates would hedge such exposure by entering into transactions which consist of either the purchase of credit default

swaps or the creation of short positions in our securities, including potentially the Notes offered hereby. Any such credit default swaps or short positions could adversely affect future trading prices of the Notes offered hereby. The underwriters and their affiliates may also make investment recommendations and/or publish or express independent research views in respect of such securities or financial instruments and may hold, or recommend to clients that they acquire, long and/or short positions in such securities and instruments.

As a consequence of their participation in the offering of Notes, the underwriters will be entitled to share in the underwriting commissions relating to the offering.

The decision to distribute the Notes hereunder and the determination of the terms of the offering were made through negotiations between us and the underwriters. None of the Lenders has been or will be involved in the decision to offer the Notes and none has been or will be involved in the determination of the terms of any distribution of the Notes.

Certain of the underwriters may not be U.S. registered broker-dealers and accordingly will not effect any sales within the United States except in compliance with applicable U.S. laws and regulations, including the rules of the FINRA.

Settlement

We expect that delivery of the Notes will be made to investors on or about , 2025, which will be business days following the date of this Prospectus Supplement (such settlement being referred to as “T+ ”). Under Rule 15c6-1 of the U.S. Exchange Act, trades in the secondary market generally are required to settle in one business day, unless the parties to any such trade expressly agree otherwise. Accordingly, purchasers who wish to trade their Notes prior to the business day immediately preceding the closing date may be required, by virtue of the fact that the Notes will settle in T+ , to specify an alternate settlement cycle at the time of any such trade to prevent a failed settlement. Purchasers of the Notes who wish to trade their Notes prior to the closing date should consult their own advisors.

No Sales or Similar Securities

We have agreed that we will not, for a period of 90 days after the date of this Prospectus Supplement, without first obtaining the prior written consent of the Representative, offer, sell, or contract to sell, directly or indirectly, any debt securities issued or guaranteed by us, except for the Notes sold to the underwriters pursuant to the underwriting agreement.

Offering Restrictions

The Notes are offered for sale pursuant to this Prospectus Supplement in those jurisdictions in the United States where it is lawful to make such offers. No action has been taken, or will be taken, which would permit a public offering of the Notes in any jurisdiction outside the United States.

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Each of the underwriters has severally represented and agreed that it has not offered, sold or delivered and it will not offer, sell or deliver, directly or indirectly, any of the Notes, in or from any jurisdiction except under circumstances that are reasonably designed to result in compliance with the applicable laws and regulations thereof.

This Prospectus Supplement qualifies the distribution of the Notes in each of the provinces of Canada solely for the purpose of registering the Notes in the United States, pursuant to the multi-jurisdictional disclosure system adopted by the United States This Prospectus Supplement does not qualify the Notes for distribution to purchasers in Canada, or to residents of Canada. Pursuant to the underwriting agreement, each underwriter has agreed that it will only, directly or indirectly, offer, sell or deliver Notes in Canada or to residents of Canada pursuant to an available exemption from Canadian prospectus requirements.

The Notes may not be acquired or held by any person who is an employee benefit plan or other plan or arrangement subject to Title I of the Employee Retirement Income Security Act of 1974, as amended (“ERISA”), or Section 4975 of the Code, or who is acting on behalf of or investing the assets of any such plan or arrangement, unless the acquisition and holding of the Notes by such person will not result in a non-exempt prohibited transaction under Section 406 of ERISA or Section 4975 of the Code.

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LEGAL MATTERS

Certain legal matters relating to Canadian law in connection with the offering of the Notes will be passed upon on behalf of the Corporation by Blake, Cassels & Graydon LLP and on behalf of the underwriters by Osler, Hoskin & Harcourt LLP. Certain legal matters relating to United States law in connection with the offering of the Notes will be passed upon on behalf of the Corporation by Paul, Weiss, Rifkind, Wharton & Garrison LLP, New York, New York and on behalf of the underwriters by Latham & Watkins LLP, Austin, Texas.

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EXPERTS

The annual consolidated financial statements of the Corporation included in this Prospectus Supplement have been audited by Ernst & Young LLP, an independent registered public accounting firm, as set forth in their report thereon, included therein, and included herein. Such consolidated financial statements have been included herein in reliance upon the report of such firm given on their authority as experts in accounting and auditing.

In connection with the audit of our annual consolidated financial statements, Ernst & Young LLP is independent with respect to TransAlta Corporation in accordance with the Rules of Professional Conduct of the Chartered Professional Accountants of Alberta and in compliance with Rule 3520 of the Public Company Accounting Oversight Board (United States).

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EXHIBIT “A”

ANNUAL AUDITED FINANCIAL STATEMENTS AS AT AND FOR THE YEARS ENDED DECEMBER 31, 2024 AND 2023

See attached.

SA-1

Consolidated Financial Statements

Management’s Report

To the Shareholders of TransAlta Corporation

Consolidated Financial Statements 

Management’s Annual Report on Internal Control Over Financial Reporting

To the Shareholders of TransAlta Corporation

 Consolidated Financial Statements

Changes in Internal Control over Financial Reporting

Consolidated Financial Statements 

Report of Independent Registered Public Accounting Firm

To the Shareholders and Board of Directors of TransAlta Corporation

Opinion on Internal Control Over Financial Reporting

We have audited TransAlta Corporation’s internal control over financial reporting as of December 31, 2024, based on criteria established in Internal Control — Integrated Framework issued by the Committee of Sponsoring Organizations of the Treadway Commission (2013 framework) (the “COSO criteria”). In our opinion, TransAlta Corporation (the “Company”) maintained, in all material respects, effective internal control over financial reporting as of December 31, 2024, based on the COSO criteria.

As indicated in the accompanying Management’s Annual Report on Internal Control over Financial Reporting, management’s assessment of and conclusion on the effectiveness of internal control over financial reporting did not include the internal controls of Heartland Generation Ltd. and Alberta Power (2000) Ltd. which are included in the 2024 consolidated financial statements of the Company and constituted 8% and 20% of total and net assets, respectively, as of December 31, 2024, and 1% and (5)% of revenues and net earnings, respectively, for the year then ended, and the equity accounted joint venture of SP Skookumchuck Investment, LLC which are included in the 2024 consolidated financial statements of the Company and constituted 1% and 6% of total and net assets, respectively, as of December 31, 2024, and 0% and 3% of revenues and net earnings, respectively, for the year then ended. Our audit of internal control over financial reporting of the Company also did not include an evaluation of the internal control over financial reporting of Heartland Generation Ltd. and Alberta Power (2000) Ltd. and the equity accounted joint venture of SP Skookumchuck Investment, LLC.

We also have audited, in accordance with the standards of the Public Company Accounting Oversight Board (United States) (“PCAOB”), the consolidated statements of financial position of TransAlta Corporation as of December 31, 2024 and 2023, the related consolidated statements of earnings, comprehensive income (loss), changes in equity and cash flows for each of the three years in the period ended December 31, 2024, and the related notes and our report dated February 19, 2025 expressed an unqualified opinion thereon.

Basis for Opinion

The Company’s management is responsible for maintaining effective internal control over financial reporting and for its assessment of the effectiveness of internal control over financial reporting included in the accompanying Management’s Annual Report on Internal Control over Financial Reporting. Our responsibility is to express an opinion on the Company’s internal control over financial reporting based on our audit. We are a public accounting firm registered with the PCAOB and are required to be independent with respect to the Company in accordance with the U.S. federal securities laws and the applicable rules and regulations of the Securities and Exchange Commission and the PCAOB.

We conducted our audit in accordance with the standards of the PCAOB. Those standards require that we plan and perform the audit to obtain reasonable assurance about whether effective internal control over financial reporting was maintained in all material respects.

Our audit included obtaining an understanding of internal control over financial reporting, assessing the risk that a material weakness exists, testing and evaluating the design and operating effectiveness of internal control based on the assessed risk, and performing such other procedures as we considered necessary in the circumstances. We believe that our audit provides a reasonable basis for our opinion.