MORAGA, Calif. — When Ben Faus went to bed at his home in the foothills above the Monterey Bay, he knew there was a chance his power would go out but he didn’t know exactly when. About 3 a.m. on Thursday he was jolted awake because his sleep apnea breathing machine stopped working.

“All of a sudden, I was like, ‘I can’t breathe,’” he said.

The decision to turn electricity off for large areas of Northern California inconvenienced and frustrated hundreds of thousands of residents, but it became increasingly dangerous for people like Mr. Faus and the state’s most vulnerable.

Around 600,000 customers were still without power on Thursday afternoon and there was no clear indication of when it might be restored. That uncertainty heightened residents’ anger as food spoiled and businesses and schools stayed closed.