Other mistakes might have led grid managers to believe more plants were affected than was the case. The Geysers geothermal power plant, about 72 miles north of San Francisco, was listed as operating at less than its usual capacity, but its owner, Calpine, said on Thursday that it had in fact been generating electricity at normal levels.
In a statement on Thursday, California I.S.O. said that some of the plants on its list were out of state and that some might have been providing power outside its system. In addition, the organization said some plants, including those that burn natural gas, might have been producing less power because it was too hot for them to operate normally.
Almost a week after the blackouts began, neither the grid operator nor state energy regulators have offered a clear and detailed explanation of why California was so short of power even though peak demand was lower than it had been during other hot days in recent years. They have broadly attributed the energy shortage on their inability to secure more electricity from other states and sources.
In a letter to Gov. Gavin Newsom late Wednesday, the heads of three agencies that oversee the state’s electricity system said they were working to determine what had gone wrong. They acknowledged that the electricity demand on Friday and Saturday — when hundreds of thousands of homes and business went dark — was “high but not above similar hot days in prior years.”
When utilities cut power to their customers, the peak demand had reached 47,000 megawatts on Friday and 45,000 on Saturday. Those were far below the highest day — 50,270 on July 24, 2006 — or the 50,116 clocked three years ago.
Perhaps even more baffling is that the agencies did not turn to the state government for help until just before the blackouts began. Had they done so, Mr. Newsom could have called on power plants that the state and municipal utilities control to generate more power or made a plea to businesses and homeowners to conserve power — steps he took on Monday after the scope of the problem became clear.
“The lack of transparency around the reality of this situation contributed to the problem,” said Carrie Bentley, chief executive officer of Gridwell Consulting and a former official at the California I.S.O.