You store electricity by pumping water uphill to the upper reservoir on sunny and windy days—and turn it back into power at night or during calm or cloudy days by letting the water flow back downhill through a turbine.Think of this system as a giant "gravity battery", where electricity is turned into gravitational potential energy and back again.That's because greenfield sites may excel in areas such as the head—the crucial difference in altitude between the two reservoirs which makes energy generation possible.We have rated each of the 1,500 bluefield sites based on cost, where top-rated options are expected to have half the capital cost of the most expensive, per unit of storage.In New South Wales, Lakes Pindari, Wyangala, Windamere, Burrendong and Chaffey all have attractive bluefield possibilities, and could support the state government's new renewable energy zones."