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Income Taxes (Tables)
 12 Months Ended
Dec. 31, 2023
Income Tax Disclosure [Abstract]  
Schedule of Effective Income Tax Rate Reconciliation
Effective income tax rate for years ended Dec. 31:
202320222021
Federal statutory rate21.0 %21.0 %21.0 %
State income tax on pretax income, net of federal tax effect6.2 6.2 6.2 
Decreases in tax from:
Plant regulatory differences (a)
(3.4)(3.8)(4.1)
Other tax credits, net NOL & tax credit allowances(0.6)(0.8)(1.0)
Other, net(0.5)(0.7)(0.4)
Effective income tax rate22.7 %21.9 %21.7 %
(a)Plant regulatory differences primarily relate to the credit of excess deferred taxes to customers through the average rate assumption method. Income tax benefits associated with the credit are offset by corresponding revenue reductions.
Schedule of Components of Income Tax Expense (Benefit)
Components of income tax expense for years ended Dec. 31:
(Millions of Dollars)202320222021
Current federal tax expense$37 $28 $17 
Current state tax expense15 9 4 
Deferred federal tax (benefit) expense(10)(5)3 
Deferred state tax (benefit) expense(3)3 6 
Deferred change in unrecognized tax expense 1 — — 
Total income tax expense$40 $35 $30 
Components of deferred income tax expense as of Dec. 31:
(Millions of Dollars)202320222021
Deferred tax (benefit) expense excluding items below$(3)$8 $18 
Amortization and adjustments to deferred income taxes on income tax regulatory assets and liabilities(10)(10)(10)
Other1 — 1 
Deferred tax (benefit) expense$(12)$(2)$9 
Schedule of Deferred Tax Assets and Liabilities
Components of the net deferred tax liability as of Dec. 31:
(Millions of Dollars)2023
2022 (a)
Deferred tax liabilities:
Difference between book and tax bases of property$350 $343 
Regulatory assets22 24 
Pension expense10 10 
Deferred fuel costs— 6 
Other7 8 
Total deferred tax liabilities$389 $391 
Deferred tax assets:
Regulatory liabilities$32 $35 
Rate refund10 3 
Environmental remediation4 4 
Other employee benefits4 3 
Tax credit carryforward3 7 
Deferred ITCs2 2 
Other4 4 
Total deferred tax assets$59 $58 
Net deferred tax liability$330 $333 
(a)Prior periods have been reclassified to conform to current year presentation.
NOL and Tax Credit Carryforwards NOL amounts represent the tax loss that is carried forward and tax credits represent the deferred tax asset. NOL and tax credit carryforwards as of Dec. 31 were as follows:
(Millions of Dollars)20232022
Federal tax credit carryforwards$3 $7 
State NOL carryforward2 2 
Summary of Statute of Limitations Applicable to Open Tax Years [Table Text Block] NSP-Wisconsin is a member of the Xcel Energy affiliated group that files a consolidated federal income tax return. Statute of limitations applicable to Xcel Energy’s consolidated federal income tax returns expire as follows:
Tax Year(s)Expiration
2014 - 2016March 2025
2020September 2024
Reconciliation of Unrecognized Tax Benefits
Unrecognized tax benefits — permanent vs. temporary:
(Millions of Dollars)Dec. 31, 2023Dec. 31, 2022
Unrecognized tax benefit — Permanent tax positions$3 $2 
Unrecognized tax benefit — Temporary tax positions— 1 
Total unrecognized tax benefit$3 $3 
Changes in unrecognized tax benefits:
(Millions of Dollars)202320222021
Balance at Jan. 1$3 $3 $2 
Additions for tax positions of prior years— — 1 
Balance at Dec. 31$3 $3 $3 
Tax Benefits Associated with NOL and Tax Credit Carryforwards
Unrecognized tax benefits were reduced by tax benefits associated with NOL and tax credit carryforwards:
(Millions of Dollars)Dec. 31, 2023Dec. 31, 2022
NOL and tax credit carryforwards$(2)$(2)
State Statute of Limitations Applicable to Open Tax Years NSP-Wisconsin is a member of the Xcel Energy affiliated group that files consolidated state income tax returns. As of Dec. 31, 2023, NSP-Wisconsin’s earliest open tax years that are subject to examination by state taxing authorities under applicable statutes of limitations are as follows:
StateTax Year(s)Expiration
Wisconsin2016-2018May 2024
Wisconsin2019October 2024