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Rate Matters (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2024
Dec. 31, 2025
Dec. 31, 2023
Dec. 31, 2022
[2]
Dec. 31, 2021
[2]
Fuel Rules [Abstract]          
Deferred fuel rules monitored costs     $ 7.2 [1] $ (8.8) $ (3.3)
Electric Rate Proceeding [Member] | PSCW [Member] | MGE [Member]          
Rate Proceedings [Abstract]          
Authorized rate increase (decrease), percentage 1.54% [3],[4]   9.01% [5]    
Authorized return on equity, percentage 9.70%   9.80%    
Approved equity capital structure, percentage 56.10%   55.60%    
Fuel Rules [Abstract]          
Fuel rules, bandwidth 2.00%   2.00%    
Electric Rate Proceeding [Member] | PSCW [Member] | Rate Matters For Future Periods [Member] | MGE [Member]          
Rate Proceedings [Abstract]          
Authorized rate increase (decrease), percentage [3],[4]   4.17%      
Authorized return on equity, percentage   9.70%      
Approved equity capital structure, percentage   56.10%      
Gas Rate Proceeding [Member] | PSCW [Member] | MGE [Member]          
Rate Proceedings [Abstract]          
Authorized rate increase (decrease), percentage 2.44% [3],[4]   0.96%    
Authorized return on equity, percentage 9.70%   9.80%    
Approved equity capital structure, percentage 56.10%   55.60%    
Gas Rate Proceeding [Member] | PSCW [Member] | Rate Matters For Future Periods [Member] | MGE [Member]          
Rate Proceedings [Abstract]          
Authorized rate increase (decrease), percentage [3],[4]   1.32%      
Authorized return on equity, percentage   9.70%      
Approved equity capital structure, percentage   56.10%      
[1] These savings will be subject to the PSCW's annual review of 2023 fuel costs, expected to be completed in 2024. MGE has proposed to return these savings over a three-month period from October 2024 through December 2024.
[2] There was no change to the recovery in the fuel rules proceedings from the amount MGE deferred.
[3] In accordance with the 2024/2025 rate order from the PSCW, MGE will have an earnings sharing mechanism, under which, if MGE earns above the authorized ROE: (i) the utility will retain 100.0% of earnings for the first 15 basis points above the authorized ROE; (ii) 50.0% of the next 60 basis points will be required to be refunded to ratepayers; and (iii) 100.0% of any remaining excess earnings will be required to be refunded to ratepayers. Th earnings calculation excludes fuel rules adjustments. See "Fuel Rules" below.
[4] The electric increase was driven by an increase in rate base including our investments made in West Riverside, local solar, and continued investment in grid modernization, as well as higher costs for transmission, pension and OPEB, and uncollectible costs (including costs previously deferred from prior years). This increase in electric costs is offset by a decrease in fuel costs and benefit from lower tax expense (including impacts from the Inflation Reduction Act). MGE will file an updated 2025 fuel forecast with the PSCW in 2024, which may impact rates in 2025, depending on any variance between the forecast submitted as part of the rates and updated forecast. In addition, the PSCW authorized MGE to defer a recovery of and a return on costs associated for any change in the in service date for Paris and Darien and force majeure costs for Badger Hollow II, Paris, and Darien that were not reflected in this rate filing. The PSCW also approved deferral of any differential in PTC tax credits reflected in rates and actual credits produced. These deferrals will be reflected in MGE's next rate case filing. The gas rate increases were also driven by our investment made in grid modernization and higher pension and OPEB and uncollectible costs (including costs previously deferred from prior years). This increase in gas costs is offset by a tax benefit related to excess deferred taxes. Included in the gas residential rate is a reduction in the customer fixed charge.
[5] The electric rate increase was driven by generation assets including our investments in Badger Hollow II (solar), Paris (solar and battery), Red Barn (wind), and West Riverside (natural gas). In addition, the reopener request included an increase in fuel costs and the recovery of deferred 2021 fuel costs. The reopener also revised the depreciation schedule for Columbia Unit 2 and shared equipment to 2029 to align with the depreciation schedule for Unit 1.