Of Southern California, NERC said, "capacity margins will remain tight. Significant amounts of imported power are required to fortify capacity margins and preserve reliability, resulting in heavily loaded transmission lines into this area during peak conditions.

"As a result, unplanned major transmission or generation outages, or extreme temperatures/demand may lead to resource constraints."

NERC, of Princeton, New Jersey, is responsible for monitoring the reliability of the power grid in the United States, Canada and parts of Mexico.

NERC said voluntary conservation and on-call interruptible loads will likely be necessary more often than usual this summer.

As always, the biggest factor in how much demand will strain the power grids in California and the United States is the weather.

NERC said nationwide, US temperatures were 12 percent warmer than normal in 2007 and 10 percent warmer than normal in 2006.

Last October, wildfires in San Diego County threatened a major transmission line and caused it to go off-line as the San Diego area nearly averted a power crisis,

More wildfires could exacerbate an already precarious situation in California this summer, NERC said.

"Drought conditions persisting in Southern California, Nevada, eastern New Mexico and western Texas currently appear to have no impact on reliability, though potential for wildfires as a result of dry conditions can threaten infrastructure and will be monitored throughout the summer months," said the NERC study.

The California Independent System Operator, manager of the power grid on which flows 80 percent of the state's electricity, forecast a summertime peak demand of 49,071 megawatts, NERC noted.

The all-time peak demand in California was reached in July 2006 at 50,270 megawatts.

Cal ISO also saw a 1-in-10 chance that hot weather will increase that peak load forecast by 3,000 megawatts, thus reducing the capacity margin to less than 10 percent without additional power purchases.

Cal ISO spokeswoman Stephanie McCorkle said the grid manager agrees with the NERC assessment that the system needs to be under a careful watch this summer for Southern California. But she said it would take an unlikely confluence of events to create blackouts.

That would mean unexpectedly low imported power from outside California, high generation and/or transmission outages on a hot day, McCorkle said.

"We have more tools in our toolbox than we have had," said Cal ISO. "We have more demand-response to tap now," said McCorkle, referring to customers that voluntarily cut use at times of high demand, often in turn for lower power rates.

In addition to high-use business customers, many residential customers have signed to have thermostats monitored remotely.