Solar hot water is great, but that's not gravitational storage, e.g. pumping water up a hill with pumps and later using the flow of water back down to generate power.

You're right it isn't, but it is storing the water on top of the house.

Yes, but I'd expect it's much lesser amounts. 80 gallons is a decent sized water heater.

ahhh i understand what you were saying now.

NZ has a lot of hydro and a reasonable amount of geo thermal generation. Solar, not so much.