Vivek is in the news again with a project based on one of my alternators. Quite a few people use them like this to charge batteries with human power, but I do have to wonder where the hours come from in the equation:
Energy (watt hours) = Power (watts) x Time (hours).
A good way to keep fit anyway.
easy pedalling for one hour at 100 watt output is good enough for lighting 10x3w LEDs for over 3 hours. (3w LED is eq to a 40watt incandescent bulb).