This intrigued me....
But is it even practical? How much power could you generate from the flow out of a running tap?

A quick google suggests 14 l/min or 0.2333 l/s flow from a tap. That's 0.0002333 m3/s. Apparently the equation for hydropower is

P = hrgk

where P is Power in kilowatts, h is height in meters, r is flow rate in cubic meters per second, g is acceleration due to gravity of 9.8 m/s2, and k is a coefficient of efficiency ranging from 0 to 1

So, assuming a drop of 5m (from the attic tank to a ground floor flat), we get

P = 5 * 0.0002333 * 9.8 * k = 0.011445k

So even for a 100% efficient turbine we'd get about 11 Watts. Just about enough to run a single low-energy lightbulb.

Damn.

[Please feel free to challenge / correct / point and laugh at my work, here]
(moon monkey is busy making memories worth repressing, Tue 15 Nov 2011, 17:30, 1 reply)

This sort of troll physics question just annoys me
My quick google came out even worse.

For the UK I found a typical flow rate of 7 l/min
If we use your 5 meter drop calculation and run the system through the most efficient micro-hydro unit I could find (running at about 58% efficiency)
you would get about 2.2 Watts which is cock all use to anyone.
( chuff monkey I AM him, Tue 15 Nov 2011, 18:27, closed)