LOS ANGELES (AP) — The head of the California Public Utilities Commission recommended Thursday that the agency delay for several months an investigation into soaring costs tied to the damaged San Onofre nuclear power plant.

At issue is who is going to pay a bill that has reached $165 million so far for repairs, inspections and replacement electricity for a plant with crippled steam generators. San Onofre hasn't produced power since Jan. 31 and it's not clear when, or if, the twin reactors will return to service.

Meeting in San Francisco, the commission postponed for the second time voting on a proposed order requiring owners Southern California Edison and San Diego Gas & Electric to disclose the potential economic hit for ratepayers from the long-running shutdown.

According to a statement, commission President Michael R. Peevey recommended the panel wait until November to initiate an investigation, after Edison is required to notify the agency if a plant has been out of service for nine months.

He said the agency is working on options for power supplies for 2013, in the event the plant remains offline into next summer.

As costs and questions about the plant's future mount, consumer and environmental advocates have been pressuring the agency to move quickly. The commission determines how much utilities can charge homeowners and businesses for electricity.

The state Division of Ratepayer Advocates sent a letter to the agency Wednesday saying only the commission "can investigate whether SCE acted reasonably, how the cost responsibility should be resolved and whether future investments to repair or replace the steam generators are justified."

The letter was co-signed by executives from The Utility Reform Network, the Alliance for Nuclear Responsibility, Friends of the Earth and the Center for Energy Efficiency and Renewable Technologies.

"We think customers shouldn't be paying a dime for units that don't work because of mistakes by the utility," said Matthew Freedman, staff attorney for The Utility Reform Network. The delays are "a discouraging sign about their willingness to hold Edison accountable."

Freedman said the investigation being proposed by Peevey would be narrower in scope than the draft order, which could be revived at the commission's Aug. 23 meeting.

The trouble began to unfold in January, when the Unit 3 reactor was shut down as a precaution after a generator tube break. Traces of radiation escaped at the time, but officials said there was no danger to workers or neighbors. Unit 2 had been taken offline earlier that month for maintenance, but investigators later found unexpected wear on scores of tubes that carry radioactive water inside both units.

Gradual wear is common in such tubing, but the rate of erosion at San Onofre startled officials since the equipment is relatively new. The four generators — two in each plant — were replaced in a $670 million overhaul and began operating in April 2010 in Unit 2 and February 2011 in Unit 3.

Overall, investigators found wear from friction and vibration in 15,000 places, in varying degrees, in 3,401 tubes inside the four generators.

A three-month federal probe blamed a botched computer analysis for generator design flaws that ultimately resulted in heavy wear to the alloy tubing. Edison has been trying to determine how to correct the problem, while environmental activists have depicted the plant as a disaster in the making.

The generators, which resemble massive steel fire hydrants, are one of the central pieces of equipment in a nuclear plant. At San Onofre, each one stands 65 feet high, weighs 1.3 million pounds, with 9,727 U-shaped tubes inside, each three-quarters of an inch in diameter.

If a tube breaks there is the potential that radioactivity could escape into the atmosphere, and serious leaks also can drain cooling water from a reactor.

The steam generators were manufactured by Japan-based Mitsubishi Heavy Industries.

The plant is owned by SCE, San Diego Gas & Electric and the city of Riverside. The Unit 1 reactor operated from 1968 to 1992, when it was shut down and dismantled.

SCE is part of Edison International, and San Diego Gas & Electric is part of Sempra Energy.