Hundreds of wildfires burned in Northern California while southern parts of the state endured an early summer heat wave.

Lightning started the more than 500 fires now burning in wilderness areas and in the heart of wine country, officials said. Much of California is still suffering after two years of drought-like conditions.

A fire west of Morgan Hill, about 40 kilometres south of San Jose, forced several residents from their homes Sunday, though officials did not have exact figures. That one covered nearly three square kilometres and was only 10 per cent contained, officials said.

Another fire spread across nearly nine square kilometres by early Sunday, after starting Saturday afternoon in Napa County and quickly moving into a mostly rural area of Solano County.

It threatened more than 100 buildings as it fed on grassy woodland about 65 kilometres southwest of Sacramento, said Roger Archey, a spokesman for the California Department of Forestry and Fire Protection.

It was 10 per cent contained Sunday morning and had destroyed one home, Archey said. A mandatory evacuation was ordered for some residents, agency spokeswoman Nancy Carniglia said.

There were as many as 75 fires in Shasta-Trinity National Forest, about 256 kilometres north of Sacramento. They ranged in size from less than 0.4 hectares to more than 300 hectares. None immediately threatened homes, Forest Service spokesman Michael Odle said.

Mendocino County had up to 90 fires, ranging in size up to 50 hectares, fire officials said.

In Southern California, the problem wasn't fires but the early summer heat wave that set record-breaking temperatures.

In some sections of California and Arizona, the mercury hit 49 C. Thousands of residents in Los Angeles were left without power Friday as the use of electricity soared to keep air conditioners working. Power was restored to 8,000 people by early Saturday.

Heat warnings for parts of central and southern California remained in effect, with the U.S. National Weather Service urging people to remain hydrated, avoid physical exertion and seek out air-conditioned places.

California Gov. Arnold Schwarzenegger directed the state Office of Emergency Services to be on standby over the weekend and reportedly called on the National Guard to help firefighters.

With files from the Associated Press