The operators of California's power grid are forecasting record electrical demand of 50,950 megawatts on Friday as the state swelters in record-breaking heat.
The California Independent System Operator has called a Flex Alert for the period between 1 p.m. and 10 p.m., when air conditioners are typically at peak use.
The grid operators are urging people to help avoid power failures by voluntarily turning off unnecessary lights, avoiding use of major appliance and by setting air conditioners to 78 degrees or higher.
The current all-time demand record is 50,270 megawatts, which was set on July 24, 2006.
High heat baking California has been compounded by monsoonal moisture, triggering thunderstorms and lightning.
Colton, a city of 53,000 about 60 miles east of Los Angeles, widely lost power Thursday morning after lightning struck the city's main electrical substation. The city's web site says the outage is expected to last eight to 10 hours.
In Los Angeles, the Department of Water and Power says crews fully restored power to 11,000 customers in Boyle Heights early Thursday and about 800 customers remain without electricity elsewhere. The number of LA customers impacted peaked at 14,000 overnight.
Forecasters say high pressure causing the heat wave is nearly stationary, so torrid conditions will continue into next week.
Risk of thunderstorms could increase if Tropical Storm Lidia south of Baja California sends moisture northward.