Power grid managers tonight ordered rolling blackouts in California for the first time since January.
Officials have called a Stage three alert and cut 500 megawatts of power - the equivalent of the power required by 500,000 homes.
A spokesman says the powercuts are being split between northern and southern California, including some in the Los Angeles area.
The California Independent System Operator, keeper of the state's electricity grid, says warmer temperatures have led to higher demand as people begin using air conditioners.
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This is combined with a lack of imported electricity from the Northwest.
It ordered California's two biggest utility companies, Pacific Gas and Electric and Southern California Edison, to cut the 500 megawatts.
PA