A Stage 3 Emergency initiates rotating power outages, according to the California ISO.
For the Flex Alert, which started an hour early Wednesday, scheduled from 2 p.m. to 9 p.m., residents were asked to use air conditioning early in the day and set thermostats at 78 in the afternoon and evening hours, while avoiding the use of major appliances between the hours of 3 and 10 p.m.
Officials were also urging businesses statewide to restrict their usage. In some cases, the state is asking business owners to support outreach to their customers about conserving energy.
On Monday, Gov. Gavin Newsom warned Californians to be prepared for more rolling blackouts over the next 72 hours -- which he called "very likely'' -- as the state struggles to meet demand for electricity during a historic, record-breaking heat wave.
RELATED: Gov. Gavin Newsom says power outages 'very likely' through Wednesday, announces changes to COVID-19 watch list
Meanwhile, an excessive heat warning -- which has been extended since it was first issued last week -- will be in effect until 9 p.m. Thursday in the San Fernando, Santa Clarita, San Gabriel and Antelope valleys, as well as the San Gabriel and Santa Monica mountains. The National Weather Service said conditions in those areas would be "dangerously hot.'' In inland Orange County and the Santa Ana Mountains, the warning was in effect until 10 p.m. Thursday.
A less serious heat advisory was in effect until 9 p.m. Thursday in the Los Angeles coastal zone -- beach cities, metropolitan Los Angeles, downtown Los Angeles and the Hollywood Hills.
LOS ANGELES POWER OUTAGES
Thousands of Angelenos went without power Tuesday night, with many continuing to be without power Wednesday morning.
By early Wednesday about 4,700 Los Angeles Department of Water & Power customers were without power in West Hills and Studio City due to a strain on the utility's distribution system caused by the heat wave.
The LA outages were caused by excessive demand and strain on the system, and were not related to Cal ISO's decision whether to impose rolling blackouts statewide.
At the height of the outage Tuesday evening, at least 12,000 customers were without power, according to DWP spokesman Joe Ramallo.
"Extreme heat and electricity demand has caused outages in parts of Los Angeles, currently affecting some LADWP customers,'' Mayor Eric Garcetti said on Twitter Tuesday night. Crews are working hard to restore power as quickly as possible.''
LADWP said power for some customers may not be restored until Wednesday afternoon. As of 4 a.m., 4,813 customers were without power, mostly in the Sun Valley, West Hills, Elysian Valley and Echo Park areas. Smaller outages were reported in Canoga Park, Panorama City, Arleta, Tujunga, Beverly Grove, Westlake and Mid-City affecting 160 or fewer customers
Repair crews took advantage of cooler temperatures overnight to make progress in restoring power to affected customers and teams will work around the clock to get customers back online, Ramallo said.
HEAT WAVE SETS DAILY RECORD
The worst day of the lingering heat wave -- the result of high pressure over Nevada -- was Tuesday, when highs reached 113 in Northridge, 112 in Van Nuys, 110 in Chatsworth, 109 at Acton and Pasadena, 108 in Saugus, 107 in Lancaster and 105 in Palmdale.
Daily records were set in Woodland Hills, where the 112-degree high broke the previous record of 109 for Aug. 18 set in 1992; at Hollywood Burbank Airport, where the 109-degree high broke the previous record of 100 set in 1986; at Long Beach Airport, where the 100-degree high was one degree higher than the previous record set in 1986; and UCLA, where the 97-degree broke the previous record of 90 set in 1986.
Inland Empire residents trying to keep cool during severe heat wave
FULL FORECAST: Heat wave continues to roast region Wednesday
In Orange County, records were set for an August 18 in Anaheim at 105 degrees, breaking the old record of 101 in 1992, and Santa Ana at 106 degrees, breaking the old record of 95 in 2010, according to the National Weather Service.
Forecasters warned that excessive heat and a heightened risk of wildfires would last at least through Thursday.
"Extreme heat will significantly increase the potential for heat related illnesses, particularly for those working or participating in outdoor activities,'' according to the weather service. Take extra precautions if you work or spend time outside. When possible reschedule strenuous activities to early morning or evening.''
The heat wave is the fourth to attack the region this summer as well as the longest and fiercest. It is caused by a strong ridge of high pressure anchored over Nevada, forecasters said.
The Associated Press and City News Service contributed to this report.