The first thing you have to ask yourself is “If capturing energy from rainfall on our roof was even vaguely practical, why don’t we hear about people doing it?”
Average *yearly* rainfall in the UK is 855 mm
The Average UK house has 33m^2 of floor space. Assuming two floors to a house this is a roof area of 16.5m^2
This gives a yearly roof capturable volume of rain of 141m^3
Which is 14,100 Litres
Which seems like a lot but is actually on 0.02 Litres / minute
Looking at a home-hydro scheme the smallest system they consider expects about 660 L / min with a static head of 25 ft producing 233W of electricity. (
http://www.pumpfundamentals.com/micro-hydro.htm)
You’re going to have to store a lot of water up on the roof (in which case your house will collapse under the extra weight) and only run your turbine very periodically.
Even assuming we could do this we could expect a typical hydro system to provide 233W for 21 minutes using a years worth of water which is 82 Wh (not kWh)
a year.
And that’s why people aren’t doing it.
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