Well, that assumes 100% efficient digestion, which isn’t correct, as I mentioned.
Also, when I was talking about expelling heat, it was in the context of keeping the body warm, per the OP. Cooling through evaporation much easier energy-wise, I believe.
What other form of energy? Electricity? Sonic? Nuclear? Assuming the guy is just sitting in a chilly room, you’ve got heat loss from skin temperature/sweat evaporation and heat loss from respiration. Sure, there are plenty of other energy expendiatures for the body, but I don’t know if any of them are temperature dependent (at least for 60F ambient vs. 70F). You’re right, you have potential energy lost from moving/expelling sweat, but what’s the difference in sweating between someone sitting in 60F vs. 70F?
Anyway, this is all just a WAG on my part. I’m sure the actual answer is nowhere close, especially seeing Squink’s winter rations figure. However, I imagine a lot of that has to do with actually doing work in cold weather as well as being exposed to thermal conditions far worse than the OP (freezing rain, wind, etc.)
I’d be interested to see someone else’s take on it.