You also have to consider the extra weight of the batteries you would be bringing along, which would eat up more of your power. The more you crunch your numbers, the easier it is to figure out why nobody else has done it. It is possible, but you would never get your money back out of it and you would need to be putting many extra hours on the stationary bike anyway. The extra food you would need to consume would also be a consideration. If you want to do this for enviro reasons, I would look into getting more efficient grow lights, like LEDs and not worrying about generating the power.