Equation often used in microhydro books is:
Average watts produced = Flow (in gpm) x Head (in feet) / 10. This assumes average efficiency in turbine, wiring, etc.
and 1 psi = 2.3 feet of head
3/4 inch pipe is only about the size of a garden hose, so you won't get high volume (gpm) before the pressure loss is too high. This depends on the length of the 3/4" pipe from where it comes off some larger pipe. But here's a quick number I found on the internet: at 9.4 gpm, there is a 10 psi drop every 100 ft in a 3/4 in pipe (think is was for iron pipe though, PVC probably different, but you get the picture).
Therefore: at 9.4 gpm w/ your pipe, and 100 ft from source, you will have ~50 psi.
So: Average watts = 9.4 gpm x 50 x 2.3 / 10 = 108 watts.
Not a lot of power....