All 1.4 million customers of San Diego Gas & Electric have had power restored, the company announced at 4:30 a.m Friday, some 13 hours after the massive outage struck.
Dave Geier, a company vice president, said 100 substations are back on line. The 100% return occured about an hour before the news conference announcement, he said.
Geier said that despite the return of power, customers should try to use electricity sparingly, lest they overload the system.
"The system is still fragile," he said.
Geier told reporters that it appears that a worker for the Arizona power agency caused the outage but it is unclear what he did. "We're not sure," he said.
The worker's action apparently caused the San Onofre nuclear power plant to go offline, officials said, along with a "cascading" impact on other sources.