Jun. 22--With the hottest days of the year upon us, state energy officials on Wednesday called for California residents to begin conserving power, saying they were surprised by a spike in energy use last week that shot above their forecast.

The state's population is increasing and more houses are being built in the hotter inland regions, but that doesn't fully explain statewide energy use on Friday that was nearly 7 percent higher than expected.

Officials say that while the state has added generating capacity since the rolling blackouts of several years ago, they are closely watching how much power is used during the next week.

"It's not until the first heat wave that we actually have the best barometer. . . . It was actually a good thing to get that heads- up," said Stephanie McCorkle, spokeswoman for the California Independent System Operator, which runs the statewide power grid.

When electricity demand exceeded expectations on Friday, statewide temperatures averaged up to 5 degrees higher than anticipated by weather forecasters and 40,319 megawatts were used as compared with the 37,726 megawatts expected, McCorkle said. That's "significant" because one megawatt is enough to provide the annual energy needs of 750 homes, she said.

Overall, energy use for the past three days has been relatively close to what grid operators forecast, but this Friday will be a sign of what to expect in the coming months, McCorkle said.

The grid operator and Pacific Gas & Electric released separate alerts Wednesday asking people to watch their energy use. Their advice ranged from closing blinds to turning off air conditioners -- requests all too familiar to customers still sensitive to the rolling blackouts that hit the state five years ago.

Thanks to new power plants and expanded transmission lines, McCorkle and PG&E spokesman Jeff Smith said there is no threat of blackouts this summer. Still, the use of power is expected to be high in Southern California, where the amount of energy available will be "tight."

That means residents in Northern California should keep their use to a minimum to reduce the strain on the statewide grid, Smith said.

The Bay Area may be facing seven days of high 80s and low 90s temperatures, but some residents farther away from the coast will dealing with thermometers reaching as high as 112.

Contact Sarah Jane Tribble at stribble@mercurynews.com or (408) 278-3499.