I've been using power output to measure my cycling workouts. It would be useful to know the incremental caloric consumption for a given average wattage over time. There should be a relationship. I'm making a guess that an average of 190 watts for an hour, is roughly equivalent to 1000 kcal. The formula to estimate kcals would be kcals= time in hours x average watts x 5.26. My guess is that this approximation would have to be adjusted for changes in physiological efficiency at different intensity levels. Is there any research available on this topic - or does someone know a better formula?