Yeah, I really don't know what the max flow rate is from 1/2" piping. If memory serves from reading that its 2.31 ft = 1 psi so 70 psi would be 160 ft equivalent to head, which would be quite good. But it is the combination of head & flow that determines the output.
50W doesn't sound like much, but that's 1200WH, or a bit better than a 200W panel getting 5.5 hrs of sunshine/day. Rain or shine. Still, I'd much rather have 1KW, 24x7..
Anyway, my reading just made me curious as to the 'potential energy' in household supply.
Obviously metering water negates any cost savings from the hypothetical question, but I'm curious how many Watts of power a microhydro turbine would be able to deliver from household water pressures of 70 PSI..
Just to give a rough answer to your question (besides the fact that it
depends on other variables like "quantity" of water per second).
I have done this frequently using a small turgo turbine. We used about
80psi (but it varied!). With a standard half inch (13mm) pipe, as is
often used to supply a garden tap, you can usually draw in the order
of one third of a litre per second without too much pressure drop.
This usually resulted in an output from a directly-driven permanent
magnet generator, of about 30 to 40 watts. Max might have been 50
watts when all was going well. The shape of the pipe feed and length
from the "mains" to the tap makes a lot of difference to the amount of
water you can draw, without too much pressure loss.
These are purely experimental results.