SAN FRANCISCO — Power has been restored after California ordered the first rolling outages in nearly 20 years when a statewide heat wave strained the electrical system.

The California Independent System Operator (California ISO), which manages the power grid, declared an emergency Friday evening and directed utilities around the state to shed their power loads.

The state’s three biggest utilities — Pacific Gas & Electric, Southern California Edison and San Diego Gas and Electric — turned off power to more than 410,000 homes and businesses for about an hour at a time until the emergency declaration ended 3 1/2 hours later.

The move came as temperatures around the state hit triple digits in many areas, and air conditioning use soared.

The power grid is mostly stressed during the late afternoon and early evening due to higher demand and solar energy production falling. The state tried to prepare for the expected rise in electricity use by urging conservation and trying to buy more power but a high-pressure system building over Western states meant there was less available.

The state remained gripped by the heat wave Saturday and the power grid operator will decide whether to continue the rolling outages on a day-to-day basis, said California ISO spokeswoman Anne Gonzales.

“We’re dealing with weather, clouds, wildfires … these are quickly evolving situations, quickly changing,” Gonzales said.

The last time the state ordered rolling outages was during an energy crisis in 2001. Blackouts occurred several times from January to May, including one that affected more than 1.5 million customers. The cause was a combination of energy shortages and market manipulation by energy wholesalers, infamously including Enron Corp., that drove up prices by withholding supplies.

Counties up and down the state reported scattered outages, although the city of Los Angeles, which has its own power generating system, wasn’t affected.

The heat wave brought dangerously high temperatures, increased wildfire danger and fears of coronavirus spread as people flock to beaches and parks for relief.

Temperatures were expected to peak above 110 degrees (43 Celsius) in inland regions of state Saturday. The sweltering weather was expected to continue into Wednesday across greater Los Angeles, the Central Valley, Sierra Nevada foothills and parts of the San Francisco Bay Area.

Santa Clara, Alameda and Contra Costa counties opened cooling centers that will welcome people this weekend from the afternoon to the early evening. San Francisco officials said the city is recommending people stay home and that if the heat indoors gets intolerable to go outside to a shady place where they can stay cool and distant from other people.

“Congregate indoor sites are not safe necessarily during COVID-19. It is better to follow other instructions during this heat wave,” said Mary Ellen Carroll, executive director of the Department of Emergency Management.

Carroll encouraged residents to check on family, friends and neighbors, especially older adults and those in frail health, and reminded people to always wear a face mask when in the vicinity of people who don’t share their household.

“We know it’s going to be beautiful out this weekend but we just want everyone to remember that we are in a very serious response to this COVID-19 virus,” Carroll said.

Several cities opened cooling centers, but with limited capacity because of social distancing requirements.

The scorching temperatures are a concern for firefighters battling blazes that have destroyed several homes and erupted near rural and urban foothill neighborhoods, driving through tinder-dry brush.

In addition to the possibility of heat stroke and other hot-weather illnesses, health officers were concerned that people will pack beaches, lakes and other recreation areas without following mask and social distancing orders — a major concern in the state that has seen more than 613,000 coronavirus cases.

Israel saw a COVID-19 resurgence after a May heat wave inspired school officials to let children remove their masks, Dr. George Rutherford, an epidemiologist at the University of California, San Francisco, told the San Francisco Chronicle.

“People will want to take off their masks when it’s hot,” Rutherford said. “Don’t do it.”