We know this after finding this website: http://www.yourgreendream.com/diy_fp_rewire.php
We spun our unit up with a battery drill to see what sort of voltage we could achieve. We measured 24Vac at 400rpm. And even more at higher speeds.
It was good to see we were getting a usable voltage out of it, but the speed needed to generate that is too high. Its unlikely that we will be able so get our pelton wheel spinning that fast.
Our plan now is to reconfigure the windings on the stator to have a higher output at lower RPM