The operator of California’s electricity grid issued a call for energy conservation Wednesday and Thursday to cope with a blistering heat wave.

With temperatures soaring past 100 degrees, the California Independent System Operator issued back-to-back “flex alerts” that urged residents and businesses to ease off on electricity use. The ISO originally set the alert for Wednesday only, but later extended it to Thursday as power usage continued to grow.

California ISO, which runs the state’s transmission grid from a nerve center in Folsom, urged Californians to turn their thermostats to 78 degrees, draw their drapes and shut off unnecessary lights and appliances. They also asked Californians to defer using major appliances until late in the evening.

The flex alert was expected to last from 2 to 9 p.m. both days.

Flex alerts are the ISO’s first line of defense against blackouts and other severe problems.

“It’s a voluntary call for conservation,” said ISO spokesman Steven Greenlee. He said the alert doesn’t mean Californians should do anything “that would threaten their health or safety.”

Demand for electricity was expected to peak at 45,867 megawatts between 4 and 6 p.m. on Wednesday and 46,888 megawatts Thursday afternoon.

Greenlee said the ISO expected to have a total of about 50,000 megawatts of power at its disposal, including standby generating units, but the grid operator is extremely reluctant to dip into its reserves. If the situation gets dire, the grid operator has the right to curtail power to industrial or commercial customers that have opted for “interruptible” electricity plans.

The temperature in Sacramento was expected to hit 105 degrees Wednesday and 107 on Thursday.