How much heat per hour do humans dissipate?

Asked by: George West


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.

Human energy equivalentThis 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