美国联合能源美股招股说明书(2025-09-25版)

Alliant Energy Corporation $725,000,000 5.750%Fixed-to-FixedReset RateJunior Subordinated Notes due 2056 Alliant Energy Corporation is offering $725 million aggregate principal amount of our 5.750%Fixed-to-FixedReset Rate Junior Subordinated Notes due 2056 The notes will bear interest (i)from and including September 26, 2025 (the “original issue date”) to, but excluding, April1, 2031 at the rate of 5.750% per annumand (ii)from and including April 1, 2031 during each Reset Period (as defined herein) at a rate per annum equal to the Five-year U.S. Treasury Rate (as defined herein)as of the most recent Reset Interest Determination Date (as defined herein) plus a spread of 2.077%, to be reset on each Reset Date (as defined herein); provided that theinterest rate during any Reset Period will not reset below 5.750% per annum (which is the same interest rate as in effect on the original issue date). Interest on the notes will accrue from and including September 26, 2025 and will be payable semi-annually in arrears on April 1 and October 1 of each year,beginning on April 1, 2026. The notes will mature on April 1, 2056. We may defer interest payments on the notes on one or more occasions for up to 10 consecutive years per deferral period as described in this prospectussupplement. Deferred interest payments will accumulate additional interest at a rate equal to the interest rate then applicable to the notes (including as reset during thedeferral period), to the extent permitted by law. At our option, we may redeem the notes at the times and at the redemption prices described in this prospectus supplement. The notes will be our general unsecured obligations and will rank junior and subordinate in right of payment to the prior payment in full of our existing and futureSenior Indebtedness (as defined herein). The notes will rank equally in right of payment with any future unsecured indebtedness that we may incur from time to time ifthe terms of such indebtedness provide that it ranks equally with the notes in right of payment. For a description of the effect of the terms of subordination applicable tothe notes, see “Description of the Notes — Subordination.” The notes are a new issue of securities with no established trading market. We do not intend to apply for the listing or trading of the notes on any securitiesexchange or trading facility or for inclusion of the notes in any automated quotation system. Investing in the notes involves risks. See “Risk Factors” on page S-8 for information on certain matters you shouldconsider before purchasing the notes. PernoteTotalPublic Offering Price(1)100.000%$725,000,000Underwriting Discount1.000%$7,250,000Proceeds to Alliant Energy Corporation (before expenses)(1)99.000%$717,750,000 (1)Plus accrued interest from September 26, 2025, if settlement occurs after that date. Neither the U.S. Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or determinedif this prospectus supplement or the accompanying prospectus is truthful or complete. Any representation to the contrary is a criminal offense. The underwriters expect to deliver the notes in book-entry form only through The Depository Trust Company, or DTC, on or about September 26,2025. Joint Book-Running Managers MUFG BofA Securities Barclays J.P.Morgan Wells Fargo Securities Table of Contents TABLE OF CONTENTS Prospectus SupplementAbout This Prospectus SupplementForward-Looking StatementsProspectus Supplement SummaryRisk FactorsUse of ProceedsCapitalizationDescription of the NotesMaterial U.S. Federal Income Tax ConsiderationsCertain ERISA ConsiderationsUnderwriting (Conflicts of Interest)Where You Can Find More InformationLegal MattersExperts Prospectus About This ProspectusForward-Looking StatementsAlliant Energy CorporationRisk FactorsUse of ProceedsDescription of Common StockDescription of Debt SecuritiesDescription of WarrantsDescription of Stock Purchase Contracts and Stock Purchase UnitsSelling ShareownersPlan of DistributionWhere You Can Find More InformationLegal MattersExperts Table of Contents ABOUT THIS PROSPECTUS SUPPLEMENT This document is in two parts. The first part is this prospectus supplement, which describes the specific terms of this offering. The second part, theaccompanying prospectus, gives more general information, some of which may not apply to this offering. You should read the entire prospectussupplement, as well as the accompanying prospectus and the documents incorporated by reference that are described under “Where You Can Find MoreInformation” in this prospectus supplement. In the event that the description of the offering varies between this prospectus supplement and theaccompanying prospectus, you should rely on the information contained in this prospectus supplement. Generally, when we refer to the “prospectus,” weare referring to this prospectus supplement and the accompanying prospectus combined. A