By Scott DiSavino June 12 (Reuters) - San Diego Gas & Electric (SDG&E) has agreed to buy power from privately held Apex Power Group's Pio Pico natural gas-fired power plant in California to maintain adequate long-term power supplies after the retirement of the San Onofre nuclear plant. "The new facility will contribute to local capacity requirements and will be instrumental in maintaining a reliable electric system," Jennifer Ramp, a spokeswoman at SDG&E, told Reuters Wednesday. The 300-megawatt (MW) Pio Pico plant in San Diego County is expected to enter service in the autumn of 2015, SDG&E said. The California Energy Commission said last year the plant would cost about $300 million to build. One megawatt can power about 800 homes. SDG&E, a unit of California power company Sempra Energy , said in a release Tuesday it has enough power to keep San Diego's air conditioners humming this summer but urged customers to be prepared to conserve energy. "While SDG&E has lined up the electric resources necessary to supply our customers with enough power this summer, in the event of an extended heat wave, we expect that we will need to ask customers to reduce their energy use," SDG&E Chairman and CEO Jessie Knight said in the release. SDG&E said it has already made plans to meet customers' energy needs for a second straight summer without San Onofre. On June 7, Southern California Edison (SCE), a unit of California power company Edison International and the majority owner and operator of the 2,150-megawatt (MW) San Onofre plant, decided to permanently retire the nuclear plant. SDG&E owns 20 percent of San Onofre. The two reactors at San Onofre shut in January 2012 following a small radioactive leak from the steam generators in one of the units, and remained out after workers discovered unusual tube wear in the steam generators. In March California utility regulators denied an application by SDG&E to buy power from Pio Pico and another plant called Quail Brush. The regulator at that time said the utility did not demonstrate a need for additional power before 2018. But that was before SCE decided to retire San Onofre. To meet its energy requirements, the state allowed SDG&E to file a new application using either Pio Pico or Quail Brush or by filing a new application, SDG&E's Ramp said. "We decided to file a new application using Pio Pico," she said, noting the company expects to file the application with regulators next week. SDG&E provides power to 1.4 million customers and natural gas to more than 860,000 customers in San Diego and southern Orange counties.