Originally Posted by MoAlpha

The power meter gives you a very good estimate of the number of calories (energy) required to do the work of a ride. What it doesn't tell you is your efficiency at producing that energy from "calories" consumed, which determines how much less fat that ride made you or how much you'll need to eat for dinner to keep from wasting away (my problem). That parameter varies across individuals and states, most importantly the proportion of aerobic and anaerobic metabolism employed. Anaerobic metabolism is much less efficient, so harder efforts use more energy. I'm sure there are other effort-related factors that I'm not aware of.

I guess I should add that heart rate is a reasonable surrogate for effort and the algorithms in our gizmos use it to estimate efficiency based on some model of its relationship to the mix of aerobic and anaerobic metabolism for an activity.