Authorities in the state of California are investigating at least 50 deaths thought to be related to the heatwave gripping parts of the US.
California authorities say the worst is not yet over
Power supplies have been stretched by the record temperatures which have reached up to 46C (115F) in some areas.
Consumers have been warned of more energy shortages although authorities say they expect the weather to turn cooler by Wednesday.
The heatwave also caused blackouts in Missouri and New York.
Many of those who died were elderly, according to authorities.
Californian power companies say energy demand peaked on Monday, rising to a record high, although they said the crisis was not yet over.
"We're asking people for one more day of conservation," said Gregg Fishman, the California Independent System Operator (Cal ISO), which manages the state's power grid.
"We're not out of the woods yet."
Cal ISO said it was still crucial for consumers to conserve energy to avoid rolling blackouts.
"It's unbreathable hot," Los Angeles office worker Shauntel Barrow told Reuters news agency. "It's hard. We were not running the air conditioning until we get home at 5 o'clock. But it was like an oven so we left it on low. We're cheating but we can't help it."
"This is a historic heat wave," Joe Desmond, undersecretary of energy affairs for the California Resources Agency, told reporters.
Mr Desmond said it was the first time in 57 years that both Northern and Southern California had endured record-breaking heat at the same time.
Electricity is now back on in many of the hundreds of thousands of homes hit by power outages in the past few days, including in California and New York.
In Queens in New York, fewer than 500 people remained without electricity on Tuesday evening according to energy suppliers, after nine days of blackout.
More than 150,000 people are still without electricity in St Louis, Missouri, where supplies were knocked out last week by storms.
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