[MrsPeaches]

Here you go:

E = mgh

1000l tank = 1000kg

7m high roof (~2 stories)

E = 1000kg * 9.81ms^-2 * 7m

=~ 70 000j

1Wh = 3600j

So a 1000l tank on your two story high roof would be able to store ~19Wh

Not including mechanical losses

For comparison a single decent sized SLA battery will be able to store 1000 Wh

[hyperpallium]

Thanks! Not as good aa I'd hoped...

[LeifCarrotson]

The numbers work better with increasing size, because volume is the cube of the dimensions. Also, you can build a pond on a hill with some excavators much more efficiently per unit volume than an enclosed plastic container.

A 1m cube weighs 1,000kg and has about 20Wh. A 10m cube weighs 1,000,000 kg and has about 20,000 Wh. A 50m deep, 1 km square reservoir has 1,000,000 Wh. And that's still assuming that it's 7m above the turbine!