California declared a state of emergency to address power system concerns, as parts of the US south-west reported dangerously high temperatures.
An excessive heat warning is in place for much of Arizona and California, and southern areas of Nevada and Utah.
People are being told to stay in air-conditioned areas and out of the sun.
Californians have also been urged to conserve energy during peak times, as temperatures are expected to remain between 37-43°C until Sunday.
Governor Gavin Newsom said the state of emergency was to “reduce the strain on the energy infrastructure and increase energy capacity”.
The California Independent System Operator, which controls most of the state’s power grid, asked people to set thermostats to 25°C or higher, avoid using major appliances and unnecessary lights.
In California’s Death Valley National Park, typically one of the hottest spots in the world, a thermometer at the Furnace Creek Visitor’s Center marked 54°C on Thursday.
Higher temperatures were felt in the San Francisco Bay Area, where several cities have set up cooling centres.
In Phoenix, Arizona, the temperatures reached 48°C, while Las Vegas reported 46°C and Denver reached 38°C for the third day in a row.
About 50 million people were under excessive heat warnings and heat advisories across the south-west.
A high-pressure system parked on the south-west since Tuesday caused the heatwave, a week before the official start of summer in the northern hemisphere.
The event is worsening a mega-drought that has dried up rivers and reservoirs. Scientists say states in the US West Coast should expect more of these events.