PHOENIX (AP) -- Arizona's biggest electric utility is going big on batteries.

Arizona Public Service Co. plans to spend hundreds of millions of dollars to add large, building-size batteries to the power grid, aiming to use capacity to store surplus power available early in the day when solar power plants are productive.

The Arizona Republic reports that the power stored in batteries then would be used in the evening when customers need the energy after solar panels power down for the night.

APS is planning for 850 megawatts of the batteries, which could supply about 212,500 homes for three to four hours. Company officials say 100 megawatts of battery capacity with four hours of storage costs about $120 million.

The Phoenix-based utility serves about 2.7 million people in 11 of Arizona's 15 counties.