The blackout was caused by a fire at a substation, according to city officials.
Power has been restored in San Francisco to most of the 130,000 homes and businesses impacted by a massive outage for several hours, the main local utility provider said Sunday.
Pacific Gas & Electric Company said in a statement on X that by 7:30 a.m. (1530 GMT), power had been restored to about 110,000 customers, with efforts being made to reconnect the remaining 21,000.
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The blackout was caused by a fire at a substation, according to city officials.
"The damage from the fire in our substation was significant and extensive, and the repairs and safe restoration will be complex. We have mobilised additional engineers and electricians," the company said.
Large parts of the West Coast tech hub, which has a population of more than 800,000 people, were plunged into darkness Saturday, with disruptions to public transport and many traffic lights not working on a busy Christmas shopping weekend.
"I know this was a rough day," San Francisco Mayor Daniel Lurie said in a video posted on X from the city's emergency operations centre.
Many traffic signals were out, leaving traffic police to manage intersections and the self-driving ride-hailing service Waymo had paused operation of its vehicles, he added.
Parts of the city were blanketed in fog, and many businesses were forced to close for the day at the height of the holiday shopping period on Saturday, the San Francisco Chronicle newspaper reported, leaving normally bustling commercial areas quiet.
The abrupt fall in shoppers just days before Christmas was "devastating" for business, a manager of Black & Gold home goods store, told the San Francisco Chronicle.