The emissivity of the human body is about 0.95 and the surface area of an average male is about 1.75 m2. Normal body temperature is 37oC but skin temperature is slightly lower, 35oC = 308 K.
In the previous Q&A we found that this typical man emits radiation at the rate of about 850 W.
If this same typical man is in surroundings that are at 25oC, he also absorbs heat. We need temperatures in units of Kelvins, 25oC = 298 K.
What is the rate at which this man absorbs heat?
The rate at which heat is emitted is given by
Rasorb = e A Tsurr4
Rasorb = (0.95)(5.67 x 10 - 8 W/m2 K4)(1.75 m2)(298 K)4
35oC = 298 K
That means the net rate is
Rnet = Remit - Rasorb
Rnet = 848 W - 740 W
Rnet = 108 W
Hence, you may already have heard that people produce about as much heat as a one-hundred watt light bulb. That is what we have just calculated.
(c) 2000, Doug Davis; all rights reserved.