There are hydroelectric plants that operate as large "batteries". These "pump storage" power plants store energy by pumping water from a lower reservoir to a higher reservoir when electricity is available and then generate electricity using the energy of water flowing from the higher to lower reservoir when the electricity is needed.

These power plants are less than 100% efficient, so they actually use more electricity than they produce! Nonetheless, they are useful and profitable because electricity is more valuable during peak demand times than it is during low demand times. Electricity can be purchased at low prices when demand is low and then sold at higher prices when demand is high.