How much energy in form of heat does a human body emit at rest level?
That's easy. The energy requirement of an average man is 2,500 Calories per day, and one Calorie is 4184J. Therefore he emits about 10.5MJ/day or about 120W.
An average woman requires 2,000 Calories per day, so she emits about 97W.
I hesitate to contradict John, but: it's simplistic to assume caloric input equals caloric output, or that caloric output is purely heat, as opposed to moving from one place to another, lifting boxes, etc. A far better model IMHO is to set up the human body as a black-body source with $\epsilon = 0.98 $ (emissivity), temperature = 310K, and some reasonable estimate as to total body area. Then you compare the absorption of heat from, say an ambient environment of 294K to see the net outflow of heat.
That does ignore conductive and convective heat flow :-) .
See, for example, the excellent calculator at hyperphysics