Today, millions of Californians are instructed to reduce their thermostats to 78 degrees.

The operator also recommended that residents pre-cool their homes to 72 degrees Fahrenheit before 4 p.m. and then set their thermostats to 78 degrees or higher during conservation hours. In addition, residents should avoid charging electric vehicles or using large appliances, it said.

"It is likely that additional flex warnings will be issued over the vacation weekend as excessive heat is forecast for most of California," the utility warned in a news release Wednesday.

The strain on the power grid increases in the late afternoon hours as air conditioning use increases while solar power supply decreases, the power grid operator said.

A heat warning is in effect for nearly 50 million people in seven Western states through Labor Day weekend, including in densely populated cities such as San Francisco and Los Angeles.

"This will be the longest and most intense stretch of heat so far this calendar year," 

And that's notable because the West has already experienced extremely high temperatures this summer, exacerbating a drought in the region that experts have linked to human-caused climate change.

California's power grid will be limited in its ability to import electricity as neighboring states anticipate high temperatures, the operator said.

"We are monitoring potential power supply shortages and changing conditions, such as wildfires or outages in power generation or transmission, that could impact supply or the grid," it said in an email.

Governor Gavin Newsom has declared a state of emergency for California.

"We anticipate that this extreme heat will be of a duration that we have not experienced in a long time," he said at a news conference.

Labor Day is expected to be the hottest day of this prolonged heat wave. On other days, temperatures are expected to be 15 to 20 degrees Fahrenheit above normal.

"Daytime high temperatures are possible in much of the world, and some locations may even set monthly records," according to the Weather Prediction Center.

'This is a deadly heat'

most of Nevada, parts of Arizona, Idaho, Utah, Oregon and Washington are also affected.

Temperatures could reach as high as 116 degrees in areas under heat warning and 105 degrees in areas under heat warning, he said.

Heat warnings are in effect for Boise, Idaho, Reno, Nevada, and Seattle.

A heat warning is in effect for cities such as Los Angeles, San Diego, Palm Springs, Fresno, San Francisco, Sacramento, and Redding in California, although not all cities are expected to see temperatures reach 116 degrees, Shackelford said.

In California, the state of emergency allows power plants to generate additional energy and gives the green light to use backup generators to reduce energy use during peak hours, according to the governor's office. It also allows ships to reduce their energy consumption.

Newsom pointed out that the extreme drought is limiting hydropower as reservoirs run low, contributing to a lack of "energy reliability," he acknowledged.

"On the supply side, we are challenged by these extremes, and on the demand side, it's not surprising that people are turning up the AC," Newsom explained. "People are understandably trying to escape the heat, so we need to address this dual challenge again."

Meanwhile, meteorologists are warning people to take the extreme heat seriously because it can be life-threatening.

"This is a deadly heat, so use common sense," said Joe Sirard of the National Weather Service in Los Angeles. "Some people do not pay close enough attention to the forecast and can put themselves in danger and die. It's very sad to talk about, of course, but people need to be aware that heat can be deadly."

Excessive heat is the leading cause of weather-related deaths in the U.S., and conditions brought on by climate change have caused extreme weather events to become deadlier and more frequent.

According to the National Weather Service, heat deaths have outpaced hurricane deaths by more than 15 to 1 over the past decade.