The California Independent System Operator, which oversees the state grid, declared a Stage 3 emergency late on Friday, initiating the outages that impacted about 750,000 homes with demand soaring from people blasting fans and air-conditioners to keep cool.

Record high temperatures were seen around the Bay Area, according to the National Weather Service, with San Francisco reaching 35 deg C and San Jose 39.4 deg C.

The intense heat is hitting at an especially vulnerable time for the region, with the pandemic forcing people to remain at home.

It’s the first time the state has declared a Stage 3 emergency since electricity shortages pushed hundreds of thousands in the region into darkness, forced California’s largest electric utility into bankruptcy and sent power prices surging to record levels in 2000 and 2001.

About half of the customers were affected as of 8.15pm local time, according to PowerOutage.us, which tracks power shutdowns.

California grid operators decided to call for the rotating outages around 6.30pm local time when they determined through a complex calculation that the state’s power reserves had fallen below a critical threshold, said Ms. Anne Gonzales, a spokesman for the California ISO.

“We had an energy shortfall,” Ms. Gonzales said in a phone interview. The state needed to reduce demand by about 1,000 megawatts, she said. That’s equivalent to the amount of power supply for about 750,000 homes, according to the California ISO, Bloomberg reported.

The bulk of the outages came from PG&E Corp. The state’s biggest utility said it expected as many as 250,000 customers to be shut off in rolling outages, with power to be fully restored by 11 pm. Customers in El Dorado and San Mateo counties were the first to be impacted, said Mr. Jeff Smith, a company spokesman.

“Unfortunately, because of the emergency nature of this, we weren’t able to notify customers in advance,” Mr. Smith said in a telephone interview. The outages occurred for 60 to 90 minutes on a rotating basis through the utility’s Northern and Central California service territory, he said.

Edison International’s Southern California Edison utility began shutting off customers shortly before 7 pm, with about 132,000 of them without power as of 7.45 pm, said spokesman David Song.

The company planned to rotate the blackouts through blocks of customers, trying to ensure no one stayed without electricity for more than an hour in the outages through 10 pm. “It’s happening pretty fast,” he said.

Sempra Energy’s San Diego Gas & Electric utility said shut-offs were “widespread” across its territory in San Diego and southern Orange counties, affecting about 58,700 customers whose power was restored as of 8 pm, an hour, and 20 minutes after the outages started.

“The California ISO is working closely with California utilities and neighboring power systems to manage the strain on the grid and to restore the power grid to full capacity,” the agency said in a statement on Friday.

California is joining regions around the world that have been grappling with extreme weather brought on by climate change in recent months. What was forecast as one of the worst heat waves in more than a century gripped parts of Europe in August? The eastern US is just emerging from July temperatures that were expected to topple daily records in Manhattan and Boston dating to the 19th century.

The rolling outages in California also come less than a year after utilities in the region deliberately cut off power to millions of customers in an effort to prevent their power lines from igniting wildfires amid unusually strong winds – another consequence of increasingly extreme weather that has led to fires across the state in the past week.

California won’t see a respite from the high temperatures until later next week as the National Weather Service forecasts a long-duration heat wave starting this weekend. The weather agency posted excessive heat warnings for much of California for Friday through Wednesday.

Electricity prices have already hit two-year highs as weather forecasters called for extreme temperatures. Spot power prices surged past US$1,000 (S$1,370) a megawatt-hour across California on Friday evening.

Natural gas prices in Southern California more than doubled on the increased need for fuel for power production, according to a report from BloombergNEF.

Grid operators will continue to monitor the situation throughout the weekend and into next week, Ms. Gonzales said. Asked whether the California ISO will need to call for additional power shut-offs, she said: “We don’t expect one, but we are prepared for one.”