One other factor to keep in mind is that the less fit you are, the less mechanical work you are able to do. For example, an untrained cyclist might be able to ride at 150W for an hour, whereas a more serious cyclist might be able to sustain 300W for an hour. The first cyclist would burn about 550 Calories, while the second would burn 1100 Calories if they both did a hard one hour ride. (More energy is burnt in recovery, but that's a lot harder to measure.)