Two natural gas-fueled electric generating plants are being planned in San Diego to replace the generating capacity lost when the San Onofre Nuclear Generating Station was mothballed.

Construction is set to begin on March 9 for the gas-fueled Pio Pico Energy Center in Otay Mesa. It will supply the electricity needs of up to 200,000 homes.

San Diego Gas & Electric Co. is seeking approval for a second plant with twice the capacity, also fueled by natural gas, which it wants to build in Carlsbad.

In addition to filling the void left by the shutdown of the nuclear plant, it would allow the utility to proceed with the planned retirement of its gas-fueled Encina Power Station, which is about 60 years old.

The nuclear plant, which had provided 20% of San Diego’s electrical power, was retired because of widespread weaknesses in the pipes of its critical cooling system.

Environmentalist groups have objected to the two gas-fired plants, arguing that the utility should have used even cleaner forms of energy production. State regulators and the California Supreme Court rejected efforts by the groups to block the utility’s plans.