California ISO calls power alert, blackouts possible

"Involuntary curtailments of service to customers including rotating blackouts are possible during this emergency declaration," the state agency said.

The California ISO, which controls most of the state's power grid initially declared a transmission emergency as high demand and power plant outages sparked some voltage problems. Within minutes it also declared a stage two power alert for southern California.

The immediate impact of the stage two alert is that utility Southern California Edison was asked to implement a program that reduces demand for power linked to air conditioning use.

The stage agency said several power plants with a total capacity of about 2,000 megawatt had tripped off-line in southern California since July 20.

One megawatt is roughly enough power for 800 homes under normal conditions. But with air conditioners laboring during a heatwave, it may not be able to supply more than 250 homes.

"We lost several generating units down there (in Southern California) throughout the course of the day," ISO spokesman Gregg Fishman said.

Southern California Edison, a unit of Edison International, said its customers were currently using 21,675 megawatts of power, a new historic record. The previous usage record of 21,112 MW was set on Wednesday with a heatwave engulfing the southern half of the state.

Fishman said the agency has not yet decided whether to ask for certain industrial and commercial customers to curtail use. These customers, known as interruptibles, receive power at a cheaper rate in return for agreeing to reduce usage when supplies are particularly tight.