How much heat per hour do humans dissipate?

Asked by: George West

Answer

The average human (according to my Coke® can's Nutrition Facts label) consumes approximately 2000 Calories per day (it's actually usually somewhere between 2200 and 4000, but 2000 is a nice number). Using a simple conversion (1000 calories = 1 Calorie, 1 calorie = 4.1868 J), this amounts to 8.37 x 10⁶ joules ingested per day.

This means that the average person expends ~8.37 x 10⁶ joules of energy per day, since most of us are in some sort of equilibrium with our surroundings. Assuming most of this energy leaves us in the form of heat, I calculate that on average we radiate ~350,000 J of energy per hour. Since Watt is just Joules per second, this is roughly equal to energy given off by a 100 Watt light bulb!

This assumption, that most of our expended energy leaves us in the form of heat, is actually a decent one. Speaking as a relatively normal college student (in all relevant respects), the amount of energy I expend doing non-thermal work on my surroundings every day seems pretty trivial. Aside from playing tennis (during and after which I _very_ actively radiate thermal energy), probably the most energetic thing I do is walk up 5 flights of stairs to my dorm room. This increase in gravitational potential energy, however, is only ~12,000 J, or on the order of 0.1% of my total energy expenditure.

The one thing I have neglected (not being a biologist/chemist/physician/whatever) is chemical and biological changes in the human body that would cause the amount of ingested energy to be different than expended energy. This could be due to things like creation/burning of fat, abnormal heat radiation (such as when the body temperature rises while the immune system combats an illness), or increased activity (such as a weightlifter building muscle mass, although I suspect that at higher levels of activity the body's efficiency drops quite a bit, and the heat/work ratio goes up).
Answered by: Gregory Ogin, Physics Undergraduate Student, UST, St. Paul, MN