Upstate ratepayers of Duke Energy Carolinas will see a 13 percent increase in their monthly electric bills if the S.C. Public Service Commission approves a request that's become common among regulated utilities this year.

The subsidiary of Charlotte-based Duke Energy is seeking to recover $145 million in unanticipated fuel expenses from the 640,000 customers it serves in areas that include Greenville, Spartanburg and Anderson counties.

Duke requested to raise rates in an annual filing with regulators on July 29. If approved, residential customers would be charged 13.2 percent more, or about $15.75 a month, for every 1,000 kilowatt hours of power they consume.

Commercial accounts would see an average increase in their bills of about 18.3 percent, and industrial customers would be charged an extra 24.4 percent.

The electricity provider said in an Aug. 1 notice that the cost overrun resulted from rising energy demand last year that increased fuel needs for power generation that coincided with a drastic jump in commodity prices.

"Each year, this 'true-up' proceeding is intended to resolve the difference between projected fuel costs and what is billed to the customer," the utility said.

Power providers in the Palmetto State account for their fuel purchases annually based on their best estimate of expenses for the following 12 months. They are considered pass-through costs, meaning they are equal to the amount that was actually spent.

A key reason some electricity providers did not charge enough for fuel last year was a run-up in the price of natural gas, which drives some power plants. The economic recovery from the COVID-19 pandemic drove up demand for the fuel, and prices followed, nearly doubling to about $5.50 per million British thermal units — the standard industry measurement — by the end of the year.

Prices have continued rising, recently settling above $8.25 because of hotter than usual weather in parts of the U.S. and Russia's invasion of Ukraine, as countries overseas scramble to make sure they have enough reliable energy.

South Carolina ratepayers of two other power companies, including another Duke subsidiary, are also subject to higher bills driven by fuel costs.

Duke Energy Progress, which serves about 175,000 customers in the northeastern part of the state, including Darlington, Florence and Sumter counties, submitted its annual fuel filing in April. It was seeking an 8.35 percent increase, or $10.15 for every 1,000 kilowatt hours.

In April, the Public Service Commission unanimously approved Dominion Energy's request for a 5.19 percent fuel-connected hike for the Richmond, Va.-based utility's 758,000 electric ratepayers in 24 counties, including the Charleston region. That works out to about $6.53 a month for residential customers using 1,000 kilowatt hours.

Santee Cooper customers won't have to worry about the fuel-spike issue for now. The state-owned utility is under a court-ordered rate freeze through 2024 as part of a lawsuit settlement.

But Santee Cooper has stated that it could file for an exemption later to be compensated for some or all of unexpected energy costs it's incurring now. It has not made a decision. Santee Cooper has trimmed its budget this year by $100 million to offset its soaring gas and coal expenses.