Every food package has a list of contents and percentages of total fat, sodium, and so on. A footnote says: “Percent daily values based on a 2,000-calorie diet.” (They mean 2,000 kcalories, but that is a different story.) That rate of energy use is equal to 100 watts. This is the rate at which we metabolize the food we eat into the work we do and the heat we liberate through our skin and respiration. Activity determines the rate of energy use. An athlete in training will use 200 watts. (The use of “watt” is simply a measure of the rate of energy use and has nothing to do with electricity.)