mrbill with 13 ft. of head and a 2" pipe you'll be drawing about 80gpm and be generating roughly 100 watts, if that's not enough a 4" pipe would atleast triple that, but like Leo said that could depend on different variables.
I'm planning a 120 volt microhydro system, the generator comes with electronic load controller and dump load coil for about $1200. I'll run a 4" pipe from above a waterfall and it'll have about 25 ft. of head. Here's a link to a good website with both 12 volt and 120 volt systems.
Let us know how your system develops, a lot of people could benefit from a low head system using the outflow from their pond.
http://www.rockyhydro.com/High_Head_Micro-Hydro.php
