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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 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 operator will decide whether to continue the rolling outages 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 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."