When you pump water over a hill you need energy to pump it up and can retrieve some of that energy when it flows down hill. That's why CA uses some of its rese

When you pump water over a hill you need energy to pump it up and can retrieve some of that energy when it flows down hill. That's why CA uses some of its reservoirs as hydroelectric storage batteries, pumping water up when electricity is cheap and letting it flow down when electricity is expensive. https://www.sdcwa.org/projects/san-vicente-pumping-facilities/ https://www.sdcwa.org/projects/san-vicente-pumping-facilitie...