A 12v 40a alternator can put out 480w
A 24v 40a alternator can put out 960w
With the smallest peg I can mount on the alternator to run on the wheel I would be able to turn the alternator approximately 50 times for every one rotation of the crank. Here is an example, he isn't using anything to store the power he is just using it to power a 220w food processor: