About 3,500 PG&E customers lost power in Benicia and Vallejo Thursday night, a PG&E spokeswoman said.
They lost power shortly after 7:30 p.m., PG&E spokeswoman Jana Morris said. PG&E crews restored power to all but 950 of those customers by about 9:15 p.m.
For 1,100 customers, the outage was caused by an underground transformer failure, Morris said. PG&E crews are still working to determine the cause for the other outages.
An additional 530 customers are expected to have power restored by 3:15 a.m.
The outage has mainly been concentrated in the areas around Rose Drive and Columbus Parkway, Morris said.