Millions of Californians had no power Sunday as Pacific Gas and Electric Co. implemented its largest-ever blackout to prevent wildfires during a fierce windstorm.

Some people may be without electricity for days longer, because even as PG&E predicted calmer weather throughout its service area by Monday, it also issued an ominous warning about another wind event that may prompt more shut-offs just one day later.

PG&E said it would try to restore as many customers as possible before the next round of extreme weather arrives Tuesday and again raises the risk that power lines could ignite a deadly wildfire. But the company conceded that it may not be able to bring electricity back for everyone in time.

“I would ask everyone to make sure you’re prepared for the potential that your power may not be restored,” said PG&E President and CEO Andy Vesey, who urged people who have their power restored between events to recharge medical equipment, phones and other electronic devices and restock emergency kits.

That means some subset of the up to 2.8 million people whom PG&E started cutting power to Saturday could continue to have no electricity until at least Wednesday — and possibly longer. PG&E must visually inspect its power lines through vehicles, helicopter and foot patrols before bringing them back online, all while looking for damage that needs to be fixed.

PG&E had not yet decided Sunday whether it would proceed with the next round of blackouts. If implemented, the shut-offs beginning Tuesday could affect between 520,000 and 640,000 customers in parts of 32 counties, all of which were part of the weekend outages as well.

It would be the fourth time PG&E turned off electricity because of fire risk in just one month.

After the company’s equipment started a series of disastrous conflagrations in 2017 and 2018, some have acknowledged that PG&E may need to take extreme measures to prevent another catastrophe.

And yet skepticism, frustration and alarm abounds as California’s largest electric utility repeatedly asks customers to go without electricity for an extended period of time.

“It's really hard to get an understanding of it,” said Andy Kruse of Orinda while shopping at the blacked-out Cole Hardware store in Oakland’s Rockridge neighborhood Sunday. “PG&E is doing everything they can but stuff happens. You wonder, is it climate change or PG&E mismanagement?”

PG&E turned off the store’s electricity on Saturday night. On Sunday, employees escorted customers around the dark store with flashlights. Cash registers were being powered by generators but everything else — the lights, wood-cutting equipment and key copiers — was suspended.

Similar scenes of disruption played out across the Bay Area. Shoppers were turned away from a Trader Joe’s near the Oakland hardware store by a sign on the door that read “Closed. We have no power.”

At UC Berkeley, administrators decided to cancel daytime classes on Monday because of the blackouts.

Should the campus cancel more classes because of power outages, students worry that they will lose their “dead week” that occurs between classes and finals because professors would need to make up instruction time.

“If this keeps happening, there wouldn’t be any study time before finals; we would just go straight from classes to finals,” said Cleo Wienbar, a senior studying political economy, who was sitting at a table in the eerily deserted Sproul Plaza on Sunday.

The scene was the most stark in the North Bay, where the dangerous Kincade Fire is burning in northern Sonoma County. As the weekend windstorm fueled the fire’s growth, more than 95,000 homes and businesses in the county also lost power because PG&E was worried about trees or branches falling on its power lines or winds breaking high-voltage electrical equipment.

That may have already happened during a separate PG&E blackout last week, when a power line the company kept on malfunctioned at the time and place where the Kincade Fire started. The cause of the fire is still under investigation.

Sonoma County Supervisor James Gore, who represents the area where the Kincade Fire is burning, said PG&E has made a good-faith effort to address the needs of local officials over the weekend.

When will the lights come back on? After your electricity goes off, it could be restored within a day or two, but be prepared for the outage to last longer. After the windy weather passes — in this case, probably Monday at 8 a.m. for most of the Bay Area, PG&E said — inspectors have to assess every line before turning power back on in each area, and they may find hazards or damage. PG&E said it would have to inspect 30,000 miles of line. Officials said the company would use 42 aircraft to conduct aerial inspections. Complicating the power restoration is a weather system that could bring high winds back to the region Tuesday. While PG&E said it would try to restore power before those winds pick up, some people might remain shut off through the second round of threatening weather. Check pge.com/pspsupdates for more information. — Rachel Swan, rswan@sfchronicle.com

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“They are taking it in the jaw, and they are still staying there working with us,” he said. “That’s what I want with everybody: Do the right damn thing and lean forward and own your place. That doesn’t mean I don’t still have my concerns. ... We are dealing with a crazy situation.”

Still, Gore said he completely agrees with Gov. Gavin Newsom’s critical description of PG&E as a historically greedy corporation that mismanaged its system for decades.

Even PG&E’s largest prolonged blackout to date may not have been enough to prevent its power lines from causing problems in the weekend’s wind. A police officer saw flames spread from the site of a fallen power pole in Lafayette in an area that was not part of PG&E’s blackout, for example.

And power was knocked out even for those not affected by PG&E’s planned shut-offs as grass fires popped up and howling wind toppled branches, trees and power lines across the region.

The city of Fremont advised its residents that it was experiencing additional power outages due to the wind, but did not specify how many. According to PG&E’s outage map, more than 1,200 customers lost power in Richmond midafternoon. More than 7,800 customers lost power on either side of the Carquinez Bridge when embers from a fire in Vallejo jumped the strait and ignited a blaze in Crockett. More than 1,900 experienced an outage in the Walnut Creek area, PG&E’s map showed. PG&E said those incidents were under investigation and did not list a cause.

On top of those blacked out deliberately, another 100,000 customers lost power because of such incidents, PG&E said.

“This weather event could be the most powerful that California has had in decades,” said PG&E spokesman Paul Doherty. “With winds of those magnitudes, it really poses a higher risk of damage to the electric system.”

By Sunday afternoon, PG&E’s blackouts extended from parts of Humboldt County to the Salinas Valley. Outages were expected to reach a portion of Kern County by Sunday night.

In the nine-county Bay Area, 1.3 million people — about 447,000 customers — were subjected to the blackouts. That’s about 1 in every 6 Bay Area residents.

The next period of windy and dry conditions should arrive in the Bay Area late Tuesday afternoon, according to Steve Anderson, a meteorologist with the National Weather Service in Monterey. Wind gusts could reach 50 to 60 mph at the highest elevations, which is less than the speeds of up to 90 mph recorded over the weekend, but still dangerous, he said.

“It will be the same areas affected,” Anderson said Sunday afternoon. “The duration will be much less than the current one we’re in.”

PG&E said Sunday night that it expects to issue an “all clear” between 6 and 8 a.m. Monday. That timing is at odds with the National Weather Service, which said it doesn’t expect to lift its red-flag warning until 11 a.m. Monday. PG&E did not return calls seeking to clarify the discrepancy.

Chronicle staff writers Catherine Ho, Anna Bauman, Carolyn Said, Joaquin Palomino and Matthias Gafni contributed to this report.

J.D. Morris is a San Francisco Chronicle staff writer. Email: jd.morris@sfchronicle.com Twitter: @thejdmorris