ALLIANT ENERGY CORP

2026 Notes - In March 2023, Alliant Energy issued $575 million of 3.875% convertible senior notes (the 2026 Notes), which are senior unsecured obligations, and used the net proceeds from the issuance for general corporate purposes. The 2026 Notes will mature on March 15, 2026 unless earlier converted or repurchased. Alliant Energy may not redeem the 2026 Notes prior to the maturity date. Holders could have converted their 2026 Notes at their option at any time prior to the close of business on the business day immediately preceding December 15, 2025 only under the following circumstances:

•during any calendar quarter commencing after the calendar quarter ending on June 30, 2023 (and only during such calendar quarter), if the last reported sale price of Alliant Energy’s common stock for at least 20 trading days (whether or not consecutive) during a period of 30 consecutive trading days ending on, and including, the last trading day of the immediately preceding calendar quarter is greater than or equal to 130% of the conversion price on each applicable trading day during such period;

•during the 5 business day period after any 10 consecutive trading day period (the “measurement period”) in which the trading price (as defined in the related Indenture) per $1,000 principal amount of 2026 Notes for each trading day of the measurement period was less than 98% of the product of the last reported sale price of Alliant Energy’s common stock and the conversion rate on each such trading day; or

•upon the occurrence of specified corporate events.

On or after December 15, 2025 until the close of business on the business day immediately preceding the maturity date, holders may convert all or any portion of their 2026 Notes at any time, regardless of the foregoing circumstances. Upon conversion of the 2026 Notes, Alliant Energy will pay cash up to the aggregate principal amount of the 2026 Notes to be converted and deliver shares of its common stock, in respect of the remainder, if any, of its conversion obligation in excess of the aggregate principal amount of the 2026 Notes being converted.

The initial conversion rate is 15.5461 shares of common stock per $1,000 principal amount of 2026 Notes (equivalent to an initial conversion price of approximately $64.32 per share of Alliant Energy’s common stock). The conversion rate is subject to adjustment in some events but will not be adjusted for any accrued and unpaid interest. In addition, following certain corporate events that occur prior to the maturity date, Alliant Energy will, in certain circumstances, increase the conversion rate for a holder who elects to convert its 2026 Notes in connection with such a corporate event.

If Alliant Energy undergoes a fundamental change (as defined in the related Indenture), then, subject to certain conditions, holders of the 2026 Notes may require Alliant Energy to repurchase for cash all or any portion of its 2026 Notes at a fundamental change repurchase price equal to 100% of the principal amount of the 2026 Notes to be repurchased, plus accrued and unpaid interest to, but excluding, the fundamental change repurchase date.

As of December 31, 2025 and 2024, the 2026 Notes were classified on Alliant Energy’s balance sheets as “Current maturities of long-term debt”. As of December 31, 2025 and 2024, the net carrying amount of the 2026 Notes was $574 million and $571 million, with unamortized debt issuance costs of $1 million and $4 million, and the estimated fair value (Level 2) of the 2026 Notes was $599 million and $591 million. As of December 31, 2025, there were 407,821 shares of Alliant Energy’s common stock related to the potential conversion of the 2026 Notes included in diluted EPS based on Alliant Energy’s average stock prices and the relevant terms of the 2026 Notes.

2028 Notes - In May 2025, Alliant Energy issued $575 million of 3.25% convertible senior notes (the 2028 Notes), which are senior unsecured obligations, and used the net proceeds from the issuance to reduce Alliant Energy’s outstanding commercial paper and for general corporate purposes. The 2028 Notes will mature on May 30, 2028 unless earlier converted or repurchased, and no sinking fund is provided for the 2028 Notes. Alliant Energy may not redeem the 2028 Notes prior to the maturity date. Holders may convert their 2028 Notes at their option at any time prior to the close of business on the business day immediately preceding March 1, 2028 only under the following circumstances:

•during any calendar quarter commencing after the calendar quarter ending on September 30, 2025 (and only during such calendar quarter), if the last reported sale price of Alliant Energy’s common stock for at least 20 trading days (whether or not consecutive) during a period of 30 consecutive trading days ending on, and including, the last trading day of the immediately preceding calendar quarter is greater than or equal to 130% of the conversion price on each applicable trading day during such period;

•during the 5 business day period after any 10 consecutive trading day period (the “measurement period”) in which the trading price (as defined in the related Indenture) per $1,000 principal amount of 2028 Notes for each trading day of the measurement period was less than 98% of the product of the last reported sale price of Alliant Energy’s common stock and the conversion rate on each such trading day; or

•upon the occurrence of specified corporate events.

On or after March 1, 2028 until the close of business on the business day immediately preceding the maturity date, holders may convert all or any portion of their 2028 Notes at any time, regardless of the foregoing circumstances. Upon conversion of the 2028 Notes, Alliant Energy will pay cash up to the aggregate principal amount of the 2028 Notes to be converted and pay or deliver, as the case may be, cash, shares of its common stock or a combination of cash and shares of its common stock, at its election, in respect of the remainder, if any, of its conversion obligation in excess of the aggregate principal amount of the 2028 Notes being converted.

The initial conversion rate is 13.1773 shares of common stock per $1,000 principal amount of 2028 Notes (equivalent to an initial conversion price of approximately $75.89 per share of Alliant Energy’s common stock). The conversion rate is subject to adjustment in some events but will not be adjusted for any accrued and unpaid interest. In addition, following certain corporate events that occur prior to the maturity date, Alliant Energy will, in certain circumstances, increase the conversion rate for a holder who elects to convert its 2028 Notes in connection with such a corporate event.

If Alliant Energy undergoes a fundamental change (as defined in the related Indenture), then, subject to certain conditions, holders of the 2028 Notes may require Alliant Energy to repurchase for cash all or any portion of its 2028 Notes at a fundamental change repurchase price equal to 100% of the principal amount of the 2028 Notes to be repurchased, plus accrued and unpaid interest to, but excluding, the fundamental change repurchase date.

As of December 31, 2025, the conditions allowing holders of the 2028 Notes to convert their 2028 Notes were not met, and the 2028 Notes were classified as “Long-term debt, net” on Alliant Energy’s balance sheet. As of December 31, 2025, the net carrying amount was $569 million, with unamortized debt issuance costs of $6 million, and the estimated fair value (Level 2) was $587 million for the 2028 Notes. As of December 31, 2025, there were no shares of Alliant Energy’s common stock related to the potential conversion of the 2028 Notes included in diluted EPS based on Alliant Energy’s average stock prices and the relevant terms of the 2028 Notes.

Five-Year Schedule of Long-term Debt Maturities -At December 31, 2025, long-term debt maturities for 2026 through 2030 were as follows (in millions):

Fair Value of Long-term Debt - Refer to Note 15 for information on the fair value of long-term debt outstanding.