An outage that began late in the morning and continued into the afternoon cut power to about 88,000 customers in the busy heart of the city, according to Pacific Gas and Electric spokesman Paul Doherty.
"We deeply apologize for the inconvenience this has caused around the city," Doherty told AFP.
"We know it was an equipment failure. It is premature to speculate on a specific cause," he said, while discussing the cause of the fire.
Traffic lights were out of service and shops plunged into darkness. But many people seemed to take the outage in stride, leaving offices or apartments to socialize on streets where traffic was more subdued than on typical work days.
Some cafes or shops continued to serve customers, with people using cash or mobile payment services such as Square that let credit or debit cards be processed through smartphones or tablets.
Public transit operator BART temporarily closed its underground metro station in the city center.
Firefighters said on Twitter that they received more than 100 calls, 20 of them related to people trapped in elevators.
By shortly after 4 pm (2300 GMT), electricity had been restored to all but approximately 3,000 customers, PG&E spokeswoman Andrea Menniti said in a message on Twitter.
The bulk of the outage lasted about seven hours.
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