As I said, this is interesting to me so I did a quick Google search.

Here is an excellent article on the subject. In this article the author explains that the body is actually horribly inefficient at converting food to energy ... about 76% of the energy is lost to things like heat. He explains that if you convert the power meter readings to calories, and then account for these inefficiencies, you get a very good approximation of calories burned to produce that power. The example he gives is a rider who maintains 300 watts for an hour has burned 1,000 calories to generate that power.

I love forums ... you learn something every day!