Exercise bikes / ergometers normally display figures including cadence, power and calories.
Cadence and power are pretty explicit, but I'm not really sure what the calories figure refers to. Is it calories consumed by the body during the session, or calories produced at the crank?
I understand that the human body is only about 25% efficient, so theres a big difference between calories burned and calories produced.
If I pedal the bike for 30 minutes at 200 watts (i.e. 200 joules/sec) then I would expect to produce 200*30*60 joules of energy, a total of 360kJ or 86 Calories. If my body is only 25% efficient at converting food energy into mechanical power at the crank, then producing 86 Calories (at the crank) actually required my body to burn 344 Calories.
After 30 minutes at 200 watts, my exercise bikes tells me I've used about 200 Calories.
This doesn't seem to match either of my calculated figures, so either my bike is faulty, or I don't understand what it's showing.