Pacific Gas and Electric Co. announced Tuesday afternoon that it will preemptively cut power for more than 800,000 customers across 34 counties in Northern California and central and coastal areas in an effort to prevent power lines from sparking wildfires during dry and windy weather.

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

What counties will be affected?

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

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

PG&E officials announced the shut-off early Tuesday afternoon, and they 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.

How long could a power shut-off last?

The duration won’t be known until the targeted shut-offs are announced, but PG&E cautions on its website that outages could last several days. Oakland and Contra Costa County officials advised residents that they could be without power as long as five days.

How does PG&E decide to issue a preemptive power shut-off?

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.

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

National Weather Service forecasters issued red-flag warnings from 5 a.m. Wednesday to 5 p.m. Thursday for the East Bay and North Bay hills and valleys, while the red-flag warning for the Santa Cruz Mountains will run from 5 p.m. Wednesday to 5 p.m. Thursday.

How should I prepare for a 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 Vanrenen 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.

Will BART service be affected by the possible outages?

Transit officials said they have been working with PG&E and don’t expect a disruption to train or station service if power is cut.

Will my cell phone still work?

Cell phone carriers said service should still work during PG&E power shut-offs. Most of California’s cell phone towers have backup generators. AT&T, Sprint, Verizon, T-Mobile and U.S. Cellular told the federal government last month they were prepared to maintain service. For sites without generators, companies also have fleets of mobile sites ready to roll. However, if a fire breaks out, companies said they can’t guarantee they’ll be able to access cell towers to recharge backup generators or deploy mobile cell sites, according to federal filings.

What should I do with food in my refrigerator?

Meat, poultry, fish and eggs should always be stored in temperatures 40 degrees Fahrenheit or colder, and frozen goods should be 0 degrees or colder, according to Department of Agriculture officials. Officials advise people to keep refrigerator and freezer doors closed as much as possible during outages. Refrigerators can safely store food — and keep items cold — for about four hours. Freezers can keep their temperature for about two days if unopened. Dry ice or blocks of regular ice can help store food, too.

How can I get updates if I’m not a PG&E account holder?

PG&E has an alert system that notifies non-account holders of power outages in a specific area. Company officials said they will try to alert people 48 hours, 24 hours and just before power outages. To sign up, visit pge.com/pspszipcodealerts.

— Michael Cabanatuan, Alejandro Serrano and J.D. Morris