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).
Gregory Ogin, Physics Undergraduate Student, UST, St. Paul, MN