Money that California utility companies save under the new federal tax overhaul will be returned to their customers in the form of lower rates, state regulators said Tuesday.

The tax law signed by President Trump in December, which slashed the top corporate tax rate from 35 percent to 21 percent, has already prompted utility regulators in states from Oklahoma to Maryland to discuss returning some of the money to customers. The California Public Utilities Commission on Tuesday followed suit.

“At the CPUC’s direction, all of the electric and gas utilities in California are tracking the savings from the tax law changes and will be required to refund the savings to their customers,” commission spokeswoman Terrie Prosper wrote in an email.

The commission typically sets rates for each of California’s three large, investor-owned utilities in multiyear proceedings that take into account many of the companies’ expenses, not just their taxes. But Prosper said that in this case, “given the size of the savings from these tax law changes, the CPUC may take action to refund the money to customers sooner.”

The exact impact this will have on bills is unclear, as is the precise size of the savings for Pacific Gas and Electric Co., Southern California Edison and San Diego Gas & Electric Co. For example, while PG&E, the state’s largest utility, faced a statutory federal income tax rate of 35 percent in 2016, a filing with the U.S. Securities and Exchange Commission showed the company paid an effective tax rate of 3.8 percent after taking advantage of various deductions.

PG&E on Tuesday estimated the annual tax savings could be roughly $500 million. The savings for individual customers, however, could be offset by increased expenses the utility expects to face in dealing with winter storms as well as trees killed by drought and the bark beetle infestation.

“These costs are on a similar order of magnitude as the benefits of tax reform over the next couple of years,” PG&E spokeswoman Lynsey Paulo wrote in an email. “We expect to work collaboratively with the CPUC to determine the best way to pass the tax savings along to customers. We have historically worked closely with Commission staff to stabilize rates and bills by using expected cost reductions like these to offset expected cost increases.”

The commission’s comments came after state Sen. Jerry Hill, who chairs a subcommittee that deals with utility issues, sent the commission a letter Tuesday saying the companies stood to reap “untold savings” from the federal tax changes. He urged the commission to figure out how much the utilities will save and lower customers’ monthly bills accordingly.

“I urge you to ensure any savings are returned to California’s ratepayers,” Hill wrote.

David R. Baker is a San Francisco Chronicle staff writer. Email: dbaker@sfchronicle.com Twitter: @DavidBakerSF