As San Diegans start to crank their air conditioners, the city and the San Diego County Water Authority are developing a way to store energy by using water.  Water officials said it’s expected to save ratepayers money in the long run. The city and authority want to build an Energy Storage Facility at the San Vicente Reservoir.  Right now, the reservoir is an emergency water storage for San Diego.  It holds enough water to last 500,000 households an entire year. “Which would be vital if we couldn’t get water into the region,” said SDCWA Energy Program Manager Kelly Rodgers.