The mayors of San Francisco, Oakland and San Jose have banded together to urge the California Public Utilities Commission to reject a proposal they fear could substantially raise the fees that people pay when switching their electricity provider from Pacific Gas & Electric Co. to a city-run program like CleanPowerSF.

On Tuesday, the mayors — London Breed in San Francisco, Libby Schaaf in Oakland and Sam Liccardo in San Jose — sent a letter to the PUC to express “substantial concerns” with a proposal the commission is scheduled to vote on Sept. 13 that could raise the fees for leaving PG&E by as much as 25 percent.

The fees are added to energy customers’ bills every month in perpetuity when they join city-run power programs to repay investor-owned utilities like PG&E for older power plants and continuing contracts to provide power.

The San Francisco Public Utilities Commission is gradually enrolling city customers in its CleanPowerSF program automatically. The program uses a cleaner mix of energy generated from renewable sources. Customers can opt out of the program, however, which would prevent them from having to pay the so-called exit fees and stay on as a PG&E customer.

But the proposal to raise the exit fees, introduced by state PUC Commissioner Carla Peterman, “would reduce our investments in long-term renewable resources ... and hinder our efforts in local development and customer programs,” the mayors wrote, adding that the costs would be unfairly borne by lower-income customers “in disadvantaged communities.”

“CleanPowerSF is central to San Francisco’s efforts to reduce our greenhouse gas emissions and make our electricity 100 percent carbon-free by 2030,” Breed said in a separate statement. “This proposal makes it more expensive for San Franciscans to choose clean energy over dirty fossil fuels.”

— Dominic Fracassa

Email: cityinsider@sfchronicle.com dfracassa@sfchronicle.com Twitter: @sfcityinsider @dominicfracassa