Understandably, this is not going to save the planet or signifigantly reduce your electric bill but it will give you really intense workout. It just seemed to make good sense that an alternator would work well since it is already voltage-regulated and cheap & easy to obtain.
It is important to get the alternator spinning before you energize the rotating field or it will be very hard to get it up to speed. The amount of power you can get depends on how strong your legs are and the size of the inverter you use. I doubt the alternator could be maxed out with human power. Of course, you could modify the alternator if you wanted to. I've gotten 160 amps before out of a modified Delcotron. This one puts out 35 amps.
Now, I realize some people would ask whats the point of all this. Well, whats the point of a stationary bike anyways? You expend energy for nothing as it goes nowhere. At least you could run a TV, stereo, lights, or whatever while you're at your exercise routine.
Some of you auto electric experts might want to point out the seemingly high voltage and say "Hey, it should be no more than 13.8 volts" but this is a $3.50 volt meter!