About the same as a 100 Watt light bulb. There is no relation to skin exposed, but there is a relation to total skin area. Exposure of skin doesn't matter because the heat eventually gets through any clothing. The energy released can more than triple with exercise. There are standard engineering formulas to calculate all this. Get all the details in the Ergonomic Design Course Notes at Cornell University that explains quite clearly how you can do the calculations.
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