I was going to look up the surface area of humans and use body temperature to calculate the watts we use. Turns out "what's the surface area of a human?" isn't as straight forward as I thought.

It occurs to me that knowing the energy that must be radiated and the temperature we do it at should let us calculate the area humans must be, except I bet with convection and conduction to keep track of, that's not so straight forward either.

Maybe if I put a person in an evacuated bell-jar and do measurements from outside....