San Diego County is under an excessive heat warning through Friday, according to the National Weather Service.
The warning, effective through 10 p.m. Friday, was issued because of a dangerous combination of high temperatures and high humidity.
Because of the potential for triple-digit heat across the state, California ISO issued a Flex Alert and called for voluntary electricity conservation from 2 to 9 p.m.
Temperatures could reach triple digits in some valleys and foothill communities, NBC 7 weather anchor Whitney Southwick said.
"The mountains will see some afternoon clouds but not much chance of showers or thunderstorms. Highs will top out in the 90s and in the deserts, we will have excessive, dangerous heat well above 110 degrees," he said.
The peak use estimate for Tuesday is 48,000 megawatts, which would be the highest demand so far this year, according to Cal ISO.
Those working or exercising outside should take precautions to avoid heat illness or exhaustion.
In San Diego County, residents may see high temperatures between 90 to 103 degrees.
San Diego Unified School District has instructed staff to follow an early-release schedule for the nearly 60 schools that have less than 80 percent of their campuses air conditioned.
The City of Poway closed its popular hiking trails to dissuade people from hiking in the heat of the day.
Even coastal areas were under a heat advisory with temperatures expected in the mid-80s. Downtown residents could see temperatures in the low 90s.
On Monday, high temperatures across the state included 115 degrees in Palm Springs, 111 in the Woodland Hills area of Los Angeles, 112 at Stockton in the northern San Joaquin Valley and 110 in several Sacramento Valley locations and at Paso Robles in the Central Coast's San Luis Obispo County.
Forecasters said the heat wave would last until midweek, and possibly into the Labor Day weekend.
To avoid heat exhaustion, stay in an air-conditioned area during the hottest hours of the day.
For those people who live without air conditioning, the county has made "Cool Zones" available. Here's a list of current locations that will provide relief from the heat. They include libraries and recreational centers across the county.
Other tips to avoid heat exhaustion include taking cool showers, drinking plenty of water and avoiding any unnecessary work or exercise outside.
Never leave kids or pets unattended in cars.
Dizziness, nausea, confusion, and headache are signs of heat stroke or exhaustion.
If someone begins to show these signs, move them to a shaded area, cool them by fanning them or spraying them with cool water. Do not give the victim fluids and call for help.