California has urged residents to cut power use as a searing heatwave settles over the state and stretches power supplies to a breaking point, in the latest sign of extreme weather conditions in the US west.

Temperatures in the most populous state are forecast to climb to well above 100F (38C) during the afternoon.

To prevent power outages, state officials asked residents and businesses to turn off lights and appliances and preset their thermostats to 78F (26C), especially during the critical hours between 4 and 9pm local time when demand typically peaks and solar power generation beings to ebb.

It’s been a brutal summer so far in the US west, where wildfires and severe drought have emerged as a growing threat. As spells of excessive heat become more frequent, the strain on power and water utilities will become more acute, scientists say. Just yesterday, the federal government ordered water cutbacks across several western states to protect the beleaguered Colorado river.

California’s grid operator made similar power usage requests during the summers and falls over the last two years, when the region experienced several bouts of record-breaking hot weather. Power systems withstood heat waves in 2021 but rolling blackouts for two days in August 2020 left about 400,000 households without power.

On Wednesday, the California grid operator projected power demand would peak at 44,919 megawatts (MW), the highest since September 2020 when usage hit 47,236 MW. One megawatt can power around 1,000 US homes on a typical day, but only about half that on a hot summer day.

As a result, power prices during the day in southern California and in Washington state could climb to their highest points since September 2021.

The heat on Wednesday is also raising the risk of wildfires. The Wishon fire, a 350-acre blaze in the Sequoia national forest, was 35% contained

On Tuesday, the US government warned that more drastic cuts in water usage were needed to protect dwindling supplies held in reservoirs that are crucial to the well-being of seven western states.

Two reservoirs – Lake Mead and Lake Powell – have fallen to just above one-quarter of their capacity this summer. If they fall much lower, the lakes will be unable to generate hydroelectric power for millions on the west, authorities say.

In Big Bar, an unincorporated area of northern California for instance, temperatures could reach 110F (43C) on Wednesday while farther north, residents in Electric City, Washington, could see 105F, the National Weather Service said.