Gravity is a conservative force, so if you raise something up really high, dropping it will recover the energy used to pull it up. Of course in practice there's

Gravity is a conservative force, so if you raise something up really high, dropping it will recover the energy used to pull it up. Of course in practice there's a lot of other things that cause energy loss, but there are lot of dams in the world that exist as "stored power", by pumping water uphill, then getting energy back as the water goes through the dam later. Big bucket high up is basically a battery. The question then becomes about the details of, like, using the power when the water gets pumped down (if you had two canals, you could probably time filling one with draining the other?). There's a lot of practical things to consider