Quick calculation: 1000L tank on a roof which is ~4m high. E = m * g * h = 1000 * 9.8 * 4 = ~40,000J Which is about 12Wh not taking into account any losses.

Quick calculation: 1000L tank on a roof which is ~4m high. E = m * g * h = 1000 * 9.8 * 4 = ~40,000J Which is about 12Wh not taking into account any losses. You could maybe improve this by having the water fall into a well but still nowhere near what a battery can provide for a fraction of the cost/complexity. A standard 100Ah battery will provide 1200Wh.