Comment by 8bitsrule

At best a 'gravity battery' is as efficient as hydroelectric, since pumped-storage is a gravity battery. Towers, trains, spinning flywheels are high-maintenance by comparison. The technology for pumped-storage requires a supply of water and a high place to pump it to. Used around the world (Europe & US, at least) since 1890s.

Bath County, VA: capacity 24GWh, since 1985

https://en.wikipedia.org/wiki/Pumped-storage_hydroelectricit...

Dinorwig Power Station, Wales: capacity 9.1GWh

https://www.theregister.co.uk/2016/05/16/geeks_guide_electri...

Worldwide, today

https://en.wikipedia.org/wiki/List_of_pumped-storage_hydroel...