Tour de France riders generate 500 watts for hours on end up the steepest mountains. What about averagely-fit mortals down at the gym?
I estimate that over an hour or so I probably expend around 300 (kilo)calories - spread over jogging, the rowing machine, static bike plus 25 minutes working the resistance machines.
At 1 kjoule = a quarter of a kcal, this is equivalent to one third of a kilowatt - averaged over the hour.
I was surprised to be working at such a rate. However, Wikipedia quotes 500 watts for an averagely-fit person working hard physically, and 900 watts as the peak power output of the non-athlete.
The Tour guys can spin up to two and a half kilowatts in a sprint finish after hours on the road ...
Note: one pound of fat is equivalent to 3,500 calories. This means - based on 2,000 to 2,500 calories per day just for being alive - that a day of fasting will burn around half a pound of fat.
As we saw above, even strenuous exercise makes very little contribution to calorie-burning.