[whswhs]

The book The Mechanics of Pre-industrial Technology has a table of power output for human muscular effort in various forms, including treadmills (horrid drudgery!). You could probably get a close estimate of power output from a suitably geared generator from that; generators aren't 100% efficient, of course, but their power losses in operation are small. I think that sustained human work output runs around 20 watts, but I would have to research that to verify.