Debt

12 Months Ended

Dec. 31, 2023

Debt Instrument [Line Items]

Debt

DEBT(a) Short-term Debt - Alliant Energy and its subsidiaries maintain committed bank lines of credit to provide short-term borrowing flexibility and back-stop liquidity for commercial paper outstanding. At December 31, 2023, the short-term borrowing capacity under a single credit facility agreement totaled $1 billion ($450 million for Alliant Energy at the parent company level, $150 million for IPL and $400 million for WPL). Subject to certain conditions, Alliant Energy (at the parent company level), IPL and WPL may each reallocate and change its sublimit up to $500 million, $400 million and $500 million, respectively, within the $1 billion total commitment. Information regarding Alliant Energy’s, IPL’s and WPL’s commercial paper classified as short-term debt was as follows (dollars in millions):

	Alliant Energy		IPL		WPL
December 31	2023	2022	2023	2022	2023	2022
Amount outstanding	$475	$642	$—	$—	$318	$290
Weighted average interest rates	5.5%	4.6%	N/A	N/A	5.4%	4.5%
Available credit facility capacity	$525	$358	$150	$100	$82	$110

	Alliant Energy		IPL		WPL
For the year ended	2023	2022	2023	2022	2023	2022
Maximum amount outstanding (based on daily outstanding balances)	$793	$665	$70	$—	$349	$325
Average amount outstanding (based on daily outstanding balances)	$386	$411	$2	$—	$157	$153
Weighted average interest rates	5.2%	2.1%	5.3%	—%	5.1%	1.6%

In January 2024, Alliant Energy, IPL and WPL extended their single credit facility agreement, which currently expires in December 2028, and reallocated credit facility capacity amounts to $350 million for Alliant Energy at the parent company level, $150 million for IPL and $500 million for WPL, within the $1 billion total commitment.

(b) Long-Term Debt - Long-term debt, net as of December 31 was as follows (dollars in millions):

	2023			2022
	Alliant Energy	IPL	WPL	Alliant Energy	IPL	WPL
Senior Debentures (a):
3.25%, due 2024	$500	$500	$—	$500	$500	$—
3.4%, due 2025	250	250	—	250	250	—
5.5%, due 2025	50	50	—	50	50	—
4.1%, due 2028	500	500	—	500	500	—
3.6%, due 2029	300	300	—	300	300	—
2.3%, due 2030	400	400	—	400	400	—
5.7%, due 2033 (b)	300	300	—	—	—	—
6.45%, due 2033	100	100	—	100	100	—
6.3%, due 2034	125	125	—	125	125	—
6.25%, due 2039	300	300	—	300	300	—
4.7%, due 2043	250	250	—	250	250	—
3.7%, due 2046	300	300	—	300	300	—
3.5%, due 2049	300	300	—	300	300	—
3.1%, due 2051	300	300	—	300	300	—
	3,975	3,975	—	3,675	3,675	—
Debentures (a):
3.05%, due 2027	300	—	300	300	—	300
3%, due 2029	350	—	350	350	—	350
1.95%, due 2031	300	—	300	300	—	300
3.95%, due 2032	600	—	600	600	—	600
4.95% due 2033 (c)	300	—	300	—	—	—
6.25%, due 2034	100	—	100	100	—	100
6.375%, due 2037	300	—	300	300	—	300
7.6%, due 2038	250	—	250	250	—	250
4.1%, due 2044	250	—	250	250	—	250
3.65%, due 2050	350	—	350	350	—	350
	3,100	—	3,100	2,800	—	2,800
Other:
AEF term loan credit agreement through March 2024, 6% at December 31, 2023 (with Alliant Energy as guarantor) (d)	300	—	—	400	—	—
AEF 1.4% senior notes, due 2026 (with Alliant Energy as guarantor) (a)	200	—	—	200	—	—
Alliant Energy 3.875% convertible senior notes, due 2026 (e)	575	—	—	—	—	—
AEF 4.25% senior notes, due 2028 (with Alliant Energy as guarantor) (a)	300	—	—	300	—	—
AEF 5.95% senior notes, due 2029 (with Alliant Energy as guarantor) (a)(f)	300	—	—	—	—	—
AEF 3.6% senior notes, due 2032 (with Alliant Energy as guarantor) (a)	350	—	—	350	—	—
Sheboygan Power, LLC 5.06% senior secured notes, due 2024 (secured by the Sheboygan Falls Energy Facility and related assets) (a)	9	—	—	17	—	—
AEF 3.75% senior notes (with Alliant Energy as guarantor) (Retired in 2023)	—	—	—	400	—	—
Other, 1% at December 31, 2023, due 2024 to 2025	—	—	—	1	—	—
	2,034	—	—	1,668	—	—
Subtotal	9,109	3,975	3,100	8,143	3,675	2,800
Current maturities	(809)	(500)	—	(408)	—	—
Unamortized debt issuance costs	(54)	(21)	(19)	(45)	(21)	(19)
Unamortized debt (discount) and premium, net	(21)	(9)	(11)	(22)	(8)	(11)
Long-term debt, net (g)	$8,225	$3,445	$3,070	$7,668	$3,646	$2,770

(a)Contains optional redemption provisions which, if elected by the issuer at its sole discretion, could require material redemption premium payments by the issuer. The redemption premium payments under these optional redemption provisions are variable and dependent on applicable U.S. Treasury rates at the time of redemption.

(b)In September 2023, IPL issued $300 million of 5.7% senior debentures due 2033. The net proceeds from the issuance were used to reduce cash amounts received from its sales of accounts receivable program, reduce commercial paper classified as long-term debt, for general corporate purposes and/or were placed in money market fund investments.

(c)In March 2023, WPL issued $300 million of 4.95% debentures due 2033. The debentures were issued as green bonds, and an amount equal to or in excess of the net proceeds was disbursed for the development and acquisition of its solar EGUs.

(d)In January 2023, AEF entered into a $300 million interest rate swap maturing in January 2026 to mitigate interest rate risk. Under the terms of the swap, AEF exchanged a variable interest rate for a fixed interest rate of 3.93% on a portion of its variable-rate term loan borrowings. In December 2023, AEF retired the remaining $100 million variable-rate term loan borrowings. Refer to Note 15 for additional information on the interest rate swap.

(e)Refer to “Convertible Senior Notes” below for additional information.

(f)In November 2023, AEF issued $300 million of 5.95% senior notes due 2029. The net proceeds from AEF’s issuance were used to reduce Alliant Energy’s outstanding commercial paper and for general corporate purposes.

(g)There were no significant sinking fund requirements related to the outstanding long-term debt.

Convertible Senior Notes - In March 2023, Alliant Energy issued $575 million of 3.875% convertible senior notes (the Notes), which are senior unsecured obligations, and used the net proceeds from the issuance for general corporate purposes. The Notes will mature on March 15, 2026 unless earlier converted or repurchased. Alliant Energy may not redeem the Notes prior to the maturity date. Holders may convert their 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 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 Notes at any time, regardless of the foregoing circumstances. Upon conversion of the Notes, Alliant Energy will pay cash up to the aggregate principal amount of the 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 Notes being converted.

The initial conversion rate is 15.5461 shares of common stock per $1,000 principal amount of 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 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 Notes may require Alliant Energy to repurchase for cash all or any portion of its Notes at a fundamental change repurchase price equal to 100% of the principal amount of the Notes to be repurchased, plus accrued and unpaid interest to, but excluding, the fundamental change repurchase date.

As of December 31, 2023, the conditions allowing holders of the Notes to convert their Notes were not met, and as a result, the Notes were classified as “Long-term debt, net” on Alliant Energy’s balance sheet. As of December 31, 2023, the net carrying amount of the Notes was $568 million, with unamortized debt issuance costs of $7 million, and the estimated fair value (Level 2) of the Notes was $572 million. As of December 31, 2023, there were no shares of Alliant Energy’s common stock related to the potential conversion of the Notes included in diluted EPS based on Alliant Energy’s average stock prices and the relevant terms of the Notes.

Five-Year Schedule of Long-term Debt Maturities - At December 31, 2023, long-term debt maturities for 2024 through 2028 were as follows (in millions):

	2024	2025	2026	2027	2028
IPL	$500	$300	$—	$—	$500
WPL	—	—	—	300	—
AEF	309	—	200	—	300
Alliant Energy parent company	—	—	575	—	—
Alliant Energy	$809	$300	$775	$300	$800

Fair Value of Long-term Debt - Refer to Note 16 for information on the fair value of long-term debt outstanding.

IPL [Member]

Debt Instrument [Line Items]

Debt

DEBT(a) Short-term Debt - Alliant Energy and its subsidiaries maintain committed bank lines of credit to provide short-term borrowing flexibility and back-stop liquidity for commercial paper outstanding. At December 31, 2023, the short-term borrowing capacity under a single credit facility agreement totaled $1 billion ($450 million for Alliant Energy at the parent company level, $150 million for IPL and $400 million for WPL). Subject to certain conditions, Alliant Energy (at the parent company level), IPL and WPL may each reallocate and change its sublimit up to $500 million, $400 million and $500 million, respectively, within the $1 billion total commitment. Information regarding Alliant Energy’s, IPL’s and WPL’s commercial paper classified as short-term debt was as follows (dollars in millions):

	Alliant Energy		IPL		WPL
December 31	2023	2022	2023	2022	2023	2022
Amount outstanding	$475	$642	$—	$—	$318	$290
Weighted average interest rates	5.5%	4.6%	N/A	N/A	5.4%	4.5%
Available credit facility capacity	$525	$358	$150	$100	$82	$110

	Alliant Energy		IPL		WPL
For the year ended	2023	2022	2023	2022	2023	2022
Maximum amount outstanding (based on daily outstanding balances)	$793	$665	$70	$—	$349	$325
Average amount outstanding (based on daily outstanding balances)	$386	$411	$2	$—	$157	$153
Weighted average interest rates	5.2%	2.1%	5.3%	—%	5.1%	1.6%

In January 2024, Alliant Energy, IPL and WPL extended their single credit facility agreement, which currently expires in December 2028, and reallocated credit facility capacity amounts to $350 million for Alliant Energy at the parent company level, $150 million for IPL and $500 million for WPL, within the $1 billion total commitment.

(b) Long-Term Debt - Long-term debt, net as of December 31 was as follows (dollars in millions):

	2023			2022
	Alliant Energy	IPL	WPL	Alliant Energy	IPL	WPL
Senior Debentures (a):
3.25%, due 2024	$500	$500	$—	$500	$500	$—
3.4%, due 2025	250	250	—	250	250	—
5.5%, due 2025	50	50	—	50	50	—
4.1%, due 2028	500	500	—	500	500	—
3.6%, due 2029	300	300	—	300	300	—
2.3%, due 2030	400	400	—	400	400	—
5.7%, due 2033 (b)	300	300	—	—	—	—
6.45%, due 2033	100	100	—	100	100	—
6.3%, due 2034	125	125	—	125	125	—
6.25%, due 2039	300	300	—	300	300	—
4.7%, due 2043	250	250	—	250	250	—
3.7%, due 2046	300	300	—	300	300	—
3.5%, due 2049	300	300	—	300	300	—
3.1%, due 2051	300	300	—	300	300	—
	3,975	3,975	—	3,675	3,675	—
Debentures (a):
3.05%, due 2027	300	—	300	300	—	300
3%, due 2029	350	—	350	350	—	350
1.95%, due 2031	300	—	300	300	—	300
3.95%, due 2032	600	—	600	600	—	600
4.95% due 2033 (c)	300	—	300	—	—	—
6.25%, due 2034	100	—	100	100	—	100
6.375%, due 2037	300	—	300	300	—	300
7.6%, due 2038	250	—	250	250	—	250
4.1%, due 2044	250	—	250	250	—	250
3.65%, due 2050	350	—	350	350	—	350
	3,100	—	3,100	2,800	—	2,800
Other:
AEF term loan credit agreement through March 2024, 6% at December 31, 2023 (with Alliant Energy as guarantor) (d)	300	—	—	400	—	—
AEF 1.4% senior notes, due 2026 (with Alliant Energy as guarantor) (a)	200	—	—	200	—	—
Alliant Energy 3.875% convertible senior notes, due 2026 (e)	575	—	—	—	—	—
AEF 4.25% senior notes, due 2028 (with Alliant Energy as guarantor) (a)	300	—	—	300	—	—
AEF 5.95% senior notes, due 2029 (with Alliant Energy as guarantor) (a)(f)	300	—	—	—	—	—
AEF 3.6% senior notes, due 2032 (with Alliant Energy as guarantor) (a)	350	—	—	350	—	—
Sheboygan Power, LLC 5.06% senior secured notes, due 2024 (secured by the Sheboygan Falls Energy Facility and related assets) (a)	9	—	—	17	—	—
AEF 3.75% senior notes (with Alliant Energy as guarantor) (Retired in 2023)	—	—	—	400	—	—
Other, 1% at December 31, 2023, due 2024 to 2025	—	—	—	1	—	—
	2,034	—	—	1,668	—	—
Subtotal	9,109	3,975	3,100	8,143	3,675	2,800
Current maturities	(809)	(500)	—	(408)	—	—
Unamortized debt issuance costs	(54)	(21)	(19)	(45)	(21)	(19)
Unamortized debt (discount) and premium, net	(21)	(9)	(11)	(22)	(8)	(11)
Long-term debt, net (g)	$8,225	$3,445	$3,070	$7,668	$3,646	$2,770

(a)Contains optional redemption provisions which, if elected by the issuer at its sole discretion, could require material redemption premium payments by the issuer. The redemption premium payments under these optional redemption provisions are variable and dependent on applicable U.S. Treasury rates at the time of redemption.

(b)In September 2023, IPL issued $300 million of 5.7% senior debentures due 2033. The net proceeds from the issuance were used to reduce cash amounts received from its sales of accounts receivable program, reduce commercial paper classified as long-term debt, for general corporate purposes and/or were placed in money market fund investments.

(c)In March 2023, WPL issued $300 million of 4.95% debentures due 2033. The debentures were issued as green bonds, and an amount equal to or in excess of the net proceeds was disbursed for the development and acquisition of its solar EGUs.

(d)In January 2023, AEF entered into a $300 million interest rate swap maturing in January 2026 to mitigate interest rate risk. Under the terms of the swap, AEF exchanged a variable interest rate for a fixed interest rate of 3.93% on a portion of its variable-rate term loan borrowings. In December 2023, AEF retired the remaining $100 million variable-rate term loan borrowings. Refer to Note 15 for additional information on the interest rate swap.

(e)Refer to “Convertible Senior Notes” below for additional information.

(f)In November 2023, AEF issued $300 million of 5.95% senior notes due 2029. The net proceeds from AEF’s issuance were used to reduce Alliant Energy’s outstanding commercial paper and for general corporate purposes.

(g)There were no significant sinking fund requirements related to the outstanding long-term debt.

Convertible Senior Notes - In March 2023, Alliant Energy issued $575 million of 3.875% convertible senior notes (the Notes), which are senior unsecured obligations, and used the net proceeds from the issuance for general corporate purposes. The Notes will mature on March 15, 2026 unless earlier converted or repurchased. Alliant Energy may not redeem the Notes prior to the maturity date. Holders may convert their 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 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 Notes at any time, regardless of the foregoing circumstances. Upon conversion of the Notes, Alliant Energy will pay cash up to the aggregate principal amount of the 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 Notes being converted.

The initial conversion rate is 15.5461 shares of common stock per $1,000 principal amount of 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 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 Notes may require Alliant Energy to repurchase for cash all or any portion of its Notes at a fundamental change repurchase price equal to 100% of the principal amount of the Notes to be repurchased, plus accrued and unpaid interest to, but excluding, the fundamental change repurchase date.

As of December 31, 2023, the conditions allowing holders of the Notes to convert their Notes were not met, and as a result, the Notes were classified as “Long-term debt, net” on Alliant Energy’s balance sheet. As of December 31, 2023, the net carrying amount of the Notes was $568 million, with unamortized debt issuance costs of $7 million, and the estimated fair value (Level 2) of the Notes was $572 million. As of December 31, 2023, there were no shares of Alliant Energy’s common stock related to the potential conversion of the Notes included in diluted EPS based on Alliant Energy’s average stock prices and the relevant terms of the Notes.

Five-Year Schedule of Long-term Debt Maturities - At December 31, 2023, long-term debt maturities for 2024 through 2028 were as follows (in millions):

	2024	2025	2026	2027	2028
IPL	$500	$300	$—	$—	$500
WPL	—	—	—	300	—
AEF	309	—	200	—	300
Alliant Energy parent company	—	—	575	—	—
Alliant Energy	$809	$300	$775	$300	$800

Fair Value of Long-term Debt - Refer to Note 16 for information on the fair value of long-term debt outstanding.

WPL [Member]

Debt Instrument [Line Items]

Debt

DEBT(a) Short-term Debt - Alliant Energy and its subsidiaries maintain committed bank lines of credit to provide short-term borrowing flexibility and back-stop liquidity for commercial paper outstanding. At December 31, 2023, the short-term borrowing capacity under a single credit facility agreement totaled $1 billion ($450 million for Alliant Energy at the parent company level, $150 million for IPL and $400 million for WPL). Subject to certain conditions, Alliant Energy (at the parent company level), IPL and WPL may each reallocate and change its sublimit up to $500 million, $400 million and $500 million, respectively, within the $1 billion total commitment. Information regarding Alliant Energy’s, IPL’s and WPL’s commercial paper classified as short-term debt was as follows (dollars in millions):

	Alliant Energy		IPL		WPL
December 31	2023	2022	2023	2022	2023	2022
Amount outstanding	$475	$642	$—	$—	$318	$290
Weighted average interest rates	5.5%	4.6%	N/A	N/A	5.4%	4.5%
Available credit facility capacity	$525	$358	$150	$100	$82	$110

	Alliant Energy		IPL		WPL
For the year ended	2023	2022	2023	2022	2023	2022
Maximum amount outstanding (based on daily outstanding balances)	$793	$665	$70	$—	$349	$325
Average amount outstanding (based on daily outstanding balances)	$386	$411	$2	$—	$157	$153
Weighted average interest rates	5.2%	2.1%	5.3%	—%	5.1%	1.6%

In January 2024, Alliant Energy, IPL and WPL extended their single credit facility agreement, which currently expires in December 2028, and reallocated credit facility capacity amounts to $350 million for Alliant Energy at the parent company level, $150 million for IPL and $500 million for WPL, within the $1 billion total commitment.

(b) Long-Term Debt - Long-term debt, net as of December 31 was as follows (dollars in millions):

	2023			2022
	Alliant Energy	IPL	WPL	Alliant Energy	IPL	WPL
Senior Debentures (a):
3.25%, due 2024	$500	$500	$—	$500	$500	$—
3.4%, due 2025	250	250	—	250	250	—
5.5%, due 2025	50	50	—	50	50	—
4.1%, due 2028	500	500	—	500	500	—
3.6%, due 2029	300	300	—	300	300	—
2.3%, due 2030	400	400	—	400	400	—
5.7%, due 2033 (b)	300	300	—	—	—	—
6.45%, due 2033	100	100	—	100	100	—
6.3%, due 2034	125	125	—	125	125	—
6.25%, due 2039	300	300	—	300	300	—
4.7%, due 2043	250	250	—	250	250	—
3.7%, due 2046	300	300	—	300	300	—
3.5%, due 2049	300	300	—	300	300	—
3.1%, due 2051	300	300	—	300	300	—
	3,975	3,975	—	3,675	3,675	—
Debentures (a):
3.05%, due 2027	300	—	300	300	—	300
3%, due 2029	350	—	350	350	—	350
1.95%, due 2031	300	—	300	300	—	300
3.95%, due 2032	600	—	600	600	—	600
4.95% due 2033 (c)	300	—	300	—	—	—
6.25%, due 2034	100	—	100	100	—	100
6.375%, due 2037	300	—	300	300	—	300
7.6%, due 2038	250	—	250	250	—	250
4.1%, due 2044	250	—	250	250	—	250
3.65%, due 2050	350	—	350	350	—	350
	3,100	—	3,100	2,800	—	2,800
Other:
AEF term loan credit agreement through March 2024, 6% at December 31, 2023 (with Alliant Energy as guarantor) (d)	300	—	—	400	—	—
AEF 1.4% senior notes, due 2026 (with Alliant Energy as guarantor) (a)	200	—	—	200	—	—
Alliant Energy 3.875% convertible senior notes, due 2026 (e)	575	—	—	—	—	—
AEF 4.25% senior notes, due 2028 (with Alliant Energy as guarantor) (a)	300	—	—	300	—	—
AEF 5.95% senior notes, due 2029 (with Alliant Energy as guarantor) (a)(f)	300	—	—	—	—	—
AEF 3.6% senior notes, due 2032 (with Alliant Energy as guarantor) (a)	350	—	—	350	—	—
Sheboygan Power, LLC 5.06% senior secured notes, due 2024 (secured by the Sheboygan Falls Energy Facility and related assets) (a)	9	—	—	17	—	—
AEF 3.75% senior notes (with Alliant Energy as guarantor) (Retired in 2023)	—	—	—	400	—	—
Other, 1% at December 31, 2023, due 2024 to 2025	—	—	—	1	—	—
	2,034	—	—	1,668	—	—
Subtotal	9,109	3,975	3,100	8,143	3,675	2,800
Current maturities	(809)	(500)	—	(408)	—	—
Unamortized debt issuance costs	(54)	(21)	(19)	(45)	(21)	(19)
Unamortized debt (discount) and premium, net	(21)	(9)	(11)	(22)	(8)	(11)
Long-term debt, net (g)	$8,225	$3,445	$3,070	$7,668	$3,646	$2,770

(a)Contains optional redemption provisions which, if elected by the issuer at its sole discretion, could require material redemption premium payments by the issuer. The redemption premium payments under these optional redemption provisions are variable and dependent on applicable U.S. Treasury rates at the time of redemption.

(b)In September 2023, IPL issued $300 million of 5.7% senior debentures due 2033. The net proceeds from the issuance were used to reduce cash amounts received from its sales of accounts receivable program, reduce commercial paper classified as long-term debt, for general corporate purposes and/or were placed in money market fund investments.

(c)In March 2023, WPL issued $300 million of 4.95% debentures due 2033. The debentures were issued as green bonds, and an amount equal to or in excess of the net proceeds was disbursed for the development and acquisition of its solar EGUs.

(d)In January 2023, AEF entered into a $300 million interest rate swap maturing in January 2026 to mitigate interest rate risk. Under the terms of the swap, AEF exchanged a variable interest rate for a fixed interest rate of 3.93% on a portion of its variable-rate term loan borrowings. In December 2023, AEF retired the remaining $100 million variable-rate term loan borrowings. Refer to Note 15 for additional information on the interest rate swap.

(e)Refer to “Convertible Senior Notes” below for additional information.

(f)In November 2023, AEF issued $300 million of 5.95% senior notes due 2029. The net proceeds from AEF’s issuance were used to reduce Alliant Energy’s outstanding commercial paper and for general corporate purposes.

(g)There were no significant sinking fund requirements related to the outstanding long-term debt.

Convertible Senior Notes - In March 2023, Alliant Energy issued $575 million of 3.875% convertible senior notes (the Notes), which are senior unsecured obligations, and used the net proceeds from the issuance for general corporate purposes. The Notes will mature on March 15, 2026 unless earlier converted or repurchased. Alliant Energy may not redeem the Notes prior to the maturity date. Holders may convert their 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 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 Notes at any time, regardless of the foregoing circumstances. Upon conversion of the Notes, Alliant Energy will pay cash up to the aggregate principal amount of the 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 Notes being converted.

The initial conversion rate is 15.5461 shares of common stock per $1,000 principal amount of 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 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 Notes may require Alliant Energy to repurchase for cash all or any portion of its Notes at a fundamental change repurchase price equal to 100% of the principal amount of the Notes to be repurchased, plus accrued and unpaid interest to, but excluding, the fundamental change repurchase date.

As of December 31, 2023, the conditions allowing holders of the Notes to convert their Notes were not met, and as a result, the Notes were classified as “Long-term debt, net” on Alliant Energy’s balance sheet. As of December 31, 2023, the net carrying amount of the Notes was $568 million, with unamortized debt issuance costs of $7 million, and the estimated fair value (Level 2) of the Notes was $572 million. As of December 31, 2023, there were no shares of Alliant Energy’s common stock related to the potential conversion of the Notes included in diluted EPS based on Alliant Energy’s average stock prices and the relevant terms of the Notes.

Five-Year Schedule of Long-term Debt Maturities - At December 31, 2023, long-term debt maturities for 2024 through 2028 were as follows (in millions):

	2024	2025	2026	2027	2028
IPL	$500	$300	$—	$—	$500
WPL	—	—	—	300	—
AEF	309	—	200	—	300
Alliant Energy parent company	—	—	575	—	—
Alliant Energy	$809	$300	$775	$300	$800

Fair Value of Long-term Debt - Refer to Note 16 for information on the fair value of long-term debt outstanding.