How much energy in form of heat does a human body emit at rest level?
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
John Rennie's answer is correct +/- 1% or so.
Sure, you can lift something up, doing work instead of emitting heat. But you equally often lift something down, converting its potential energy to heat that your body emits. Besides, most of the loads you lift are small compared to the energy "cost" of running the chemical factory that is your body.
Or say you climb up the stairs in a 20-story building. Assuming 1 floor = 10', that's 200 ft-lb. of work, or around 270J. Let's say that takes you 2 minutes, during which your body emits 120*100 = 12,000J of heat. So the work you did amounts to only about 2% of the total energy you consumed during that stair climb. And a modern human doesn't do physical work for very much of the day, so the conversion to work amounts to less than 1% error.
Besides, you (usually) climb down about as much as you climb up -- again converting your potential energy to heat -- so that pretty much cancels out.
Just assume that it takes around 100W to run your body at idle, more if you have to do "work" (running, etc.)