State regulators on Thursday granted Pacific Gas & Electric a $120 million rate hike over the next four years to pay for the repair and maintenance of its natural gas system, but will now require the utility to report what it does with the money.

Average residential customers will see their bills go from about $51.60 per month to $51.96 this year, a roughly 0.7 percent bump. But charges could climb as high as $58.16 by 2014, according to an estimate.

The increase has been in the works since before last year’s deadly San Bruno explosion, and the utility is considered likely to seek additional rate hikes once the scope of post-San Bruno repairs becomes clear. But concern over pipeline safety pushed the California Public Utilities Commission to require the utility to begin detailing which projects it is spending the money on and why.

As part of the unanimously approved plan, PG&E will have to file the reports twice a year. Until now there was no requirement that PG&E provide a detailed accounting of how it uses money from ratepayers. Critics said that lack of follow-up allowed the utility to delay projects at will, possibly at the expense of public safety.

The first report is due Oct. 1, said Commissioner Timothy Simon. “I assure you that any issues detected with regard to PG&E’s prioritization and spending will be brought to the attention of the full commission,” he said.

The utility got $461.8 million in gas revenue in 2010 and, under the terms of the deal approved Thursday, is slated to bring in $514.2 million this year, $541.4 million in 2012, $565.1 million in 2013 and $581.8 million in 2014. That is significantly less in each year than the utility initially requested.

While the commission documents don’t offer details, they indicate the money will go primarily to work on the company’s transmission lines and storage facilities.

PG&E declined to say if other rate hikes are in the offing, but given the scope of the pipeline repair and replacement program the utility unveiled in the aftermath of the San Bruno explosion, it is almost certain to request additional increases to pay for it. The utility has proposed testing or replacing much of its older gas pipeline and is in a vigorous discussion with regulators about how far that work should go.

But PG&E spokeswoman Christine Cordner said the utility welcomes the reporting requirements.

“We’re happy to do it. It’s something we expected,” she said. “This is providing more information and making it public.”

Assemblyman Jerry Hill, D-San Mateo, who represents San Bruno, supported the commission’s move toward greater accountability, but he remains concerned ratepayers will end up on the hook for the utility’s shortcomings.

“It’s about time they hold PG&E accountable for the ratepayer money they have collected in the past,” he said. “But if it turns out their maintenance plan was negligent or ineffective, it shouldn’t be our responsibility.”

Hill said forcing ratepayers to cover the cost of fixing pipes that may have been “negligently maintained” is akin to making them pay for the costs of the explosion in San Bruno. He said customers aren’t responsible, but the company and its shareholders should be.

After the Sept. 9 explosion, critics pointed out that the utility had twice justified rate increases using a pipeline segment just a few miles north of the section that exploded. Even though PG&E said in documents to the commission that the segment of line 132 had an unacceptably high risk of failure, the pipeline work that the rate increases were granted for was not done.

Lawmakers and San Bruno residents demanded to know at a December town hall meeting what happened to the money. Utilities commission Executive Director Paul Clanon said he was sure it had been spent on pipeline work, but had no proof.

PG&E spokesman Joe Molica said the effort to replace that roughly 7,400-foot segment is under way. The utility has begun doing the engineering work and plans to break ground in 2012.

According to PUC documents, the commission also plans to tackle the training for fire crews on the locations of gas lines and their shut-off valves.

San Bruno fire Chief Dennis Haag acknowledged at a National Transportation Safety Board hearing in March that he had not known a large gas line ran through his city.

The commission also said it will take on the question of ensuring PG&E workers are dispatched quickly to ruptures. It took the utility nearly 90 minutes to stop the flow of gas to the 30-inch diameter pipe.

No word was immediately available Thursday on a timetable for rules on those issues

Contact Joshua Melvin at 650-348-4335.