Can't speak to efficiency, and obviously depends a lot on assumptions... but a big pool with a 40 m height difference stores about a gigajoule [1]. [1] https:/

Can't speak to efficiency, and obviously depends a lot on assumptions... but a big pool with a 40 m height difference stores about a gigajoule [1]. [1] https://www.wolframalpha.com/input?i=volume+of+an+olympic+swimming+pool+*+density+of+water+*+40+m+*+1+gravity https://www.wolframalpha.com/input?i=volume+of+an+olympic+sw...