San Francisco’s top elected officials and its public utilities chief are asking state regulators to investigate a series of power outages affecting the city since a major blackout on Dec. 20. In a letter to California Public Utilities Commission President John Reynolds, the officials counted 40 large-scale outages since January, each affecting more than 1,000 customers. That amounts to nearly one outage a week. More than a quarter of the outages affected the city’s west side, where the December blackout lasted the longest. The Dec. 20 outage left about one-third of San Francisco without power at its peak. The city’s letter calls for a public investigation of PG&E that examines both that blackout and the subsequent outages. The signatories are the mayor, city attorney, president of the Board of Supervisors and general manager of the San Francisco Public Utilities Commission. They also cite delayed infrastructure projects, two recent fires caused by PG&E equipment and the utility’s plan to sell Substation I, a property in the South of Market neighborhood that the city has indicated it wants to buy. The California Public Utilities Commission regulates utility companies. A broader review would put the recurring outages and the other concerns raised by city officials before the state agency responsible for oversight of PG&E.