I see this.

http://mwands.com/store/freedom-ii-hydro-pmg-permanent-magnet-generator?search=hydro

900 rpms  to charge at 48v  and is rated for 2800 watts the electric 1/4 hp motor does 1750 rpms

I just so happen to have a 48v golf cart next to 4 mining rigs running on a 230v PDU consuming 13.3 amps (~3000 watts)..  would be pretty easy to test this..

Planning on adding solar / wind eventually so will use components for that..