By Dan Whitcomb

LOS ANGELES, June 22 (Reuters) - Firefighters worked to contain some 400 wildfires burning across Northern California on Sunday as the state baked under a fourth day of an early summer heat wave that has strained the power grid and left residents wilted.

One structure was destroyed and 150 homes were evacuated near Fairfield, 40 miles (64 km) southwest of Sacramento, in the path of the worst of the fires, which blackened more than 3,500 acres (1,416 hectares) in wine-producing Napa County.

"The weather is, of course, very hot and dry here, and this fire quickly rolled up into some extremely steep terrain and became inaccessible. We're having trouble establishing control lines," said Battalion Chief David Shew of the California Department of Forestry and Fire Protection.

He said the blaze was about 10 percent contained as of Sunday morning and that crews were hoping for a break as triple-digit temperatures began to ease and cooler off-shore breezes returned.

Most of the hundreds of fires scattered across Northern California were started by dry lightning strikes during thunder storms that moved across the state on Friday.

"Those evil clouds are wreaking havoc across the state," Mike Jarvis, deputy director of the California Department of Forestry and Fire Protection, said of the dry lightning. "There's no moisture in them and when they hit it's not like they put themselves out."

In a 24-hour period beginning on Friday, some some 5,000 to 6,000 dry lightning strikes were recorded across the region, leaving crews scrambling to keep up with spot fires.

"We do have significant numbers of fires that are completely unstaffed as of yet," Shew said. "We don't have sufficient resources to send to every one of them, so they'll just have to pick them up as we can."

California has cooked for four days under a severe heat wave that has drive temperatures above 100 degrees Fahrenheit (37.8 degrees Celsius) across much of the state.

Beaches were swamped with Californians seeking relief, and in Southern California power was lost to some communities as the power grid was strained by residents turning up air conditioners and fans to beat the heat. (Editing by Cynthia Osterman)

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