On something like an excercise bike or something like that.
If we sat around and pedaled a small generator while watching TV would it be enough to seriously offset an electric bill?
How about a hamster in its wheel?
I’d say a few dozen KWH per month. Depends on the specifics of the system: gearing, flywheel mass, cell storage, etc.
2,000 kilocalories = 2,324 watt hours
So if you eat 2,000 Calories a day, you’re running at 2,324/24 = 96.8 watts.
Some of that has to go for things other than powering an exercise bike, but you can also exceed that output for short periods of time. In the steady state though, your power producing ability is limited by the number of Calories you take in.
Gilligan always managed to generate enough juice for whatever project the Professor was attempting to power. 
A rough equivalent to the hosepower (746 watts) is the “human power” which is 1/10 of a horsepower or about 75 watts.
According to the site posted by 1010011010, a human can generate a tenth of a horsepower for several hours.
Or until the knees or some other body part gives out.
Putting this into context
At my fittest, on an exercise bike, I could generate 200-250 watts - just enough to power a PC. I could NOT power it for very long - probably an hour max, sweating like a pig. I guess a professional road cyclist could probably power a PC for a day with energy to spare, but most of us could not power much.
Si
I was watching a show about human powered flight. In that show the builders were using 1/3 of a horsepower as what a strong cyclist could produce for a period of an hour or two. This is about 250 watts.
If you believe Hollywood, all of humanity can provide sufficient electricity to power a vast, world-dominating computer system. And that’s without even breaking a sweat! http://www.thebestcasescenario.com/projects/matrix_regenerator/mr_worklog/mr_images/cap2.jpg
You were fooled. The Professor brought along a compact nuclear pile and thermoelectric generator which he had buried under his hut (remember, he had the only private hut) and which provided all the power for his neferious schemes. He merely made Gilligan ride the bike to keep him engaged in his cover experiments and keep him diverted from what he was really working on.
Do not trust the Professor. He’s keeping them there for his own, as yet undiscovered purposes.
Stranger
You put me on a deserted island with Mary Ann and I’ll whip out a few kilowatts for ya.
On researching electric assisted bikes, I saw it mentioned a few places that Tour de France-league riders could do 375 watts for a little while. And the most basic electric assisted bikes seem to start out at about 400 watts input.
I ran the figures for Lance Armstrong once, and on the hilly sections, he was averaging that much just in lifting his weight against gravity alone. Plus however much he was expending on air resistance. And sustaining it for hours at a time.
Then again, Lance Armstrong is about as far as you can get from the “typical human” the OP is asking about.
here is some Info on Floyd Landis during the '05 TDF.
500 watts for four hours and that was ninth place!
As far as Armstrong’s output goes, have you seen this cite?
I think I’ll start up a gym in which all of the various treadmills and stationary cycles are geared to generators instead of flaring it all off with friction. If I turn the lights off and pass on the annoying disco music, encourage cold showers and use an abacus at the cash drawer, I can probably offer discounts on the fees in proportion to how much power an individual exerciser helps me feed back into the grid.
Surely someone at least offers equipment that has the fancy LED readouts but doesn’t need to be plugged in. Don’t they?
None of the exercise bikes I’ve ever used had to be plugged in. Electricity for them was always generated by the cyclist. Ditto for rowing machines, although I don’t have much experience with them. Also, I think elliptical trainers aren’t plugged in, although I could be wrong about that. They certainly don’t need to be.
On the other hand, treadmills and stair step trainers have to be plugged in, since the person on them is not forcing the moving parts to move.
It should be noted that the computers and displays on these machines don’t consume anything like all power a person generates. Even if you’re totally out of shape and can only do 40 rpm on the lowest resistance level, you will still produce far more power than is needed to run the electronics. So they are still throwing away most of the power produced by the ‘motor’.
I have never seen a rowing machine of a stationary bike that used the work of the person on them to power the electronics. The rowing machines I have seen the electronics were battery operated same with the bikes. The elliptical machines I have used needed to be plugged in. This seemed to be primarily to power the motor that set the incline of the machine.
Well, you’re probably right about rowing machines, but as I said, I don’t have a lot of experience with them. And, yes I’d forgotten about the elevation motors on elliptical trainers. While they could probably be run off human power, they require lots more power than electronics. They’d have to put some big capacitors in them and it’d take a little while for the human to charge them up. Or something like that.
But Lifecycle stationary cycles definitely run off human power. The display does not work unless the machine is being pedalled. Perhaps some other brand has batteries in them.