With millions of homes and businesses cranking air conditioners to cope with temperatures above 43.3°C, electricity use in the largest US state hit the highest level since 2017 on Monday evening.

“Rotating outages are a possibility today,” said Elliot Mainzer, chief executive officer of the California Independent System Operator, known as Caiso, which runs the state’s grid. To avoid blackouts, consumers will need to ramp up conservation by two or three times, Mainzer said during a media briefing. 

Caiso also called a transmission emergency in northern California, saying back-up electricity generation is needed to relieve overloads in the area. 

The prospect of outages underscores how grids have become vulnerable in the face of extreme weather as they transition from fossil fuels to renewable energy. California has aggressively closed natural-gas power plants in recent years, leaving the state increasingly dependent on solar farms that go dark late in the day just as electricity demand peaks. At the same time, the state is enduring its worst drought in 1,200 years, sapping hydropower production.

Much of California is under an excessive heat warning for the next four days. Sacramento hit a daily record high of 43.3°C,on Monday, according to the National Weather Service. It could hit 46°C on Tuesday, shattering records, Bob Oravec, a senior branch forecaster with the US Weather Prediction Center, said. Downtown Los Angeles reached 39.4°C, on Sunday.

The heat wave, which began the last week of August, is remarkable for both its ferocity and duration, officials said. 

Each day the heat drags on, the risk of power failures rises. Searing temperatures seep into concrete over time, making it increasingly difficult to cool buildings. And the longer power plants run full tilt, the more likely they are to break down.   

California officials said the state could break the all-time record for power demand on Tuesday, as students come back to classrooms and businesses reopen after the long holiday weekend. In addition, hot and dry conditions across the state will leave it vulnerable to wildfires. 

Ahead of the heat wave, California governor Gavin Newsom has issued an emergency proclamation to free up extra power supplies.

The fight to keep power flowing in California is complicated by wildfires near Los Angeles and San Diego that are threatening transmission lines and power plants, though there had been no major interruptions as of Sunday afternoon, Mainzer said. 

A break from the heat will come across southern California later this week, thanks to Tropical Storm Kay in the Pacific Ocean, Oravec said. 

Kay, set to become a hurricane later this week, is forecast to edge up the coastline of Mexico’s Baja California peninsula. As it moves north, the storm will pump moisture and clouds into Southern California and Arizona, taking an edge off the heat. BM