Pacific Gas and Electric Co. said Monday that it might preemptively cut power this week to much of Northern California — including parts of almost every Bay Area county — to prevent power lines from sparking wildfires during dry and windy weather.

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The shut-off watch, covering 29 of the state’s 58 counties, and affecting more than 600,000 customers, is unprecedented in scope and could grow as forecasts come into focus. Customers in parts of the counties could lose power as soon as Wednesday morning, and the watch extends through Thursday.

Seven of the nine Bay Area counties — all but San Francisco and Marin — were advised of the potential outage, along with the North Coast, the northern parts of the Central Valley and the northern and central Sierra and foothills. The company cited a “potentially widespread, strong and dry wind event.”

A shut-off watch precedes a warning of an actual planned outage.

Late Monday night, PG&E released estimates of how many customers in each county could be affected. Among them were 32,613 customers in Alameda; 40,219 in Contra Costa; 32,124 in Napa; 38,123 in Santa Clara; 14,766 in San Mateo; 32,862 in Solano; and 66,289 in Sonoma.

The cities that could go dark included parts of Oakland, Berkeley, Hayward, Richmond, El Cerrito, Pinole, Calistoga, American Canyon, San Mateo and many coastal communities along Highway 1, San Jose, Gilroy, Vallejo, Petaluma and many more.

Farther north, outages could take place in east Chico and throughout the Redding area, among many other parts of Northern California, the map indicates. The potential outages are not limited to fire-prone areas, since PG&E must turn off entire distribution and transmission lines to minimize the risk of wildfires.

PG&E has said the map is intended only to provide a general idea of where shut-offs could occur. The utility recommends using an address look-up tool on its website, www.pge.com, to check specific addresses.

Southern California Edison, which provides power to much of the southern part of the state, issued its own advisory Monday, saying it could cut power this week to about 12,000 customers in Kern, Inyo, Mono, Riverside and San Bernardino counties.

Dry, warm offshore winds from the northeast — known as Diablo winds — are predicted this week, and the National Weather Service issued a 36-hour fire weather watch starting 5 a.m. Wednesday for the North Bay hills and valleys, the East Bay hills, the Diablo Range and the Santa Cruz Mountains. Similar watches have been issued through most of Northern California.

Winds are expected to be 20 to 30 mph in the mountains with gusts of at least 55 mph. Gusts up to 30 mph are expected in valleys.

Here’s what you need to know about PG&E’s potential power shut-off:

What counties would be affected?

Portions of Alameda, Alpine, Amador, Butte, Calaveras, Colusa, Contra Costa, El Dorado, Glenn, Lake, Mariposa, Mendocino, Napa, Nevada, Placer, Plumas, San Joaquin, San Mateo, Santa Clara, Santa Cruz, Shasta, Sierra, Solano, Sonoma, Stanislaus, Tehama, Tuolumne, Yolo and Yuba counties could lose their electricity, PG&E said.

When will a decision be made on who will lose power?

PG&E officials said they plan to give 24 to 48 hours notice to anyone who will have power cut. The company will contact customers by automated telephone call, text message or email. Customers who have critical medical equipment powered by electricity will get in-person visits from PG&E representatives.

The utility advises customers to make sure their contact information is up to date.

How long could a power shut-off last?

The duration won’t be known until the shut-offs are announced, but PG&E cautions on its website that outages in general could last several days. PG&E cannot simply turn power back on when the weather improves. Crews must first conduct safety inspections on foot and via cars, trucks or helicopter to check for any damaged equipment, like a tree that falls on a line and knocks it over, which could start a fire.

How does PG&E decide to issue a preemptive power cutoff?

Before issuing a shut-off, PG&E considers whether red-flag fire danger warnings have been issued by the National Weather Service. Low humidity levels must generally be below 20% and high winds need to be above 25 mph with gusts in excess of 45 mph. Officials also consider the amount of dry fuel, like fallen branches on the ground, the moisture content of vegetation, and observations from PG&E field crews and meteorologists.

Learn more To see which cities and which counties could potentially go dark, go to bit.ly/possibleshutoffs.

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None of those factors alone prompts a shut-off, officials said, and the thresholds can change depending on other conditions.

Has the National Weather Service issued any red-flag warnings for this week?

Not yet, but it has issued fire weather watches for the Bay Area and most of Northern California, the Sierra and Sierra foothills. Fire weather watches generally precede red-flag warnings, which tend to be announced about 24 hours in advance.

“It’s pretty likely that as we get closer, we will put out a red-flag warning,” said Spencer Tangen, a National Weather Service meteorologist.

How should I prepare for a planned power outage?

Public safety and PG&E officials advise residents to prepare for life without electricity by:

• Keeping phones and other electronics charged while also having backup charging methods available. External batteries, solar chargers, hand-cranked chargers and car chargers are among the options.

• Building or replenishing emergency kits that include flashlights or other illumination devices, spare batteries, a first-aid kit, emergency food and water, and cash.

• Learning how to manually open your garage door.

• Unplugging electrical appliances to avoid overloading circuits and preventing fire hazards when power is restored.

I have solar panels. Will my power still be shut off?

Customers with solar systems are still connected to the PG&E power grid, so if PG&E cuts off the power for safety, their power gets shut off automatically. People who have a home battery paired with their solar system might have power saved up and be spared from the outage.

PG&E spokeswoman Ari Van Renen stressed that it’s important for customers with solar power to keep an emergency plan for extended outages.

Should I buy a generator?

Generators can be a helpful but expensive solution to a temporary problem. They can run anywhere from a couple hundred dollars to as much as $14,000. Many residential generators are the portable kind powered by natural gas, though, so they pollute the air with carbon monoxide, nitrogen oxides, sulfur dioxide and particulate matter, further worsening air quality and injecting more greenhouse gases into the atmosphere. The main thing experts agree upon: Don’t try to install your own generator if you aren’t an expert. The do-it-yourself dangers outweigh the benefit of saving a buck.

When did PG&E start public safety power shut-offs?

PG&E instituted the precautionary power shut-offs to prevent its equipment from starting wildfires in October 2018, a year after some of its power lines were blamed for starting the devastating Wine Country wildfires.

Michael Cabanatuan, Alejandro Serrano and J.D. Morris are San Francisco Chronicle staff writers. Email: mcabanatuan@sfchronicle.com, alejandro.serrano@sfchronicle.com, jd.morris@sfchronicle.com Twitter: @ctuan, @serrano_alej, @thejdmorris