Though you need less volume if it is only being used for overnight storage rather than long term rainfall storage. For example, to store enough energy to produce 1GW for 1 day (~10^5 seconds) with a head of 100m, use Pt=mgh to work out you need to store:

m = 10^9.10^5 / (10*100) = 10^11 kg of water. = 10^8 m^3

This is not a big reserviour, being 1/20th of the size of Warragamba dam's reserviour (in Sydney). One valley could provide 20GW for a day. Compare that with demand for where I live:

http://www.aemo.com.au/Electricity/Data/Price-and-Demand/Pri...

Peak demand for 7.5M people is 9GW, with an average closer to 7GW. One valley could provide overnight storage for over 20 million people, with first world demands (all of Australia). The limitation is transmission and generating/pumping capacity, rather than storage volume.

Note that's a winter peak you're looking at.

The highest summer peak appears to have been just above 13GW. http://www.wattclarity.com.au/2016/02/highest-electricity-de...

However your point still stands I believe.

But the summer peak is due to air conditioning during the day, when there's tons of "free" solar energy.

Solution: swimming pool on the roof.

http://www.fastcodesign.com/1672202/you-deserve-a-house-with...

The more sun, the more water in the pool. Win-win!

Seriously though has anyone crunched the numbers on rainwater storage for microgeneration in a domestic setting.

Could I efficiently use excess solar to pump water in to a house roof storage area and generate electric from that?

They crunch the numbers here: http://physics.ucsd.edu/do-the-math/2011/11/pump-up-the-stor...

The energy in one gallon of gasoline is equivalent to the potential energy stored in 13 tons of water one kilometer up

The Tesla Powerwall has 6.5 kwh = 23.4 M joules.

Gravity potential energy = mgh. Suppose pool is 10 meters up, then you need 239,000 kg of water = 63K gallons or 8400 cu ft. This is a huge pool: 29 ft. x 29 ft x 10 ft.

You could use the city water supply: borrow water through a turbine at night, and return it during the day :-)

Or have pool up very high, for example on a cliff overlooking the Mediterranean:

http://inhabitat.com/crazy-home-carved-into-a-coastal-cliff-...

The outflow from an Auckland homes storm water is not insignificant - an average roof would have something around 5 cubic metres of water come off it several times per year.

I remember a while ago on the show "Let's Get Inventing" (Saturday morning kids show with kids inventing stuff, think it was on TV2), they tried generating electricity from drainpipes, they couldn't get enough power to even power a servo squeezing the trigger on a spraybottle, they had to get a great big firehose from an firetruck to actually generate anything.

"Have you seen the swimming pool on the roof?"

;)