State could face rolling blackouts

California faces a small, although worrisome, chance of rolling blackouts this summer, according to the operator of most of the state’s power transmission system.

Southern California faces a 3.7% probability of a supply problem, while Northern California has a 3.5% chance for a Stage 3 power emergency, the California Independent System Operator, or Cal-ISO, said in documents on its website. A Stage 3 designation allows power managers to order rolling blackouts to maintain the stability of the electricity system.

California is coping with a growing population and rising electricity demand. Electricity-system managers avoided rolling blackouts in July when record-breaking temperatures sent power consumption to new highs. Supplies are tightest in Southern California.

Although the probability of rolling blackouts “is minimal, it is enough for us to be concerned about it,” Gregg Fishman, a Cal-ISO spokesman, said Tuesday. Rolling blackouts are “always our last resort.”

The Cal-ISO transmission system includes the territories served by Southern California Edison Co., San Diego Gas & Electric Co. and Pacific Gas & Electric Co. The Los Angeles Department of Water and Power isn’t part of the Cal-ISO transmission grid and hasn’t predicted any summer shortages.

Voluntary conservation may help maintain the stability of the power system and help prevent supply disruptions, Fishman said. The outlook for this summer is similar to that for 2006, the system operator said.

California may declare a Stage 3 power emergency when the reserve margin, a measure of surplus power supplies, falls below 3%. A Stage 2 emergency, which may occur when the reserve margin falls below 5%, allows utilities to curtail supplies to customers who have agreed to reductions during emergencies in exchange for lower rates.

A Stage 1 emergency may occur at less than 7% reserve margin and results in public calls for conservation.