You're basically correct. I don't know if you've found the Wikipedia article on Basal Metabolic Rate but it's got everything you need, including a bunch of formulas you can use to figure out an average value for a given body type. There are also many online calculators, such as this one. You can easily figure out averages, which I think is what you want, but between two individuals the number can change quite a bit.
Why does 80 watts sound so low? This is more of a physics.se answer, but Watts are a unit of power which mean they measure energy use over time; W=J/s, or Joules per second. Every second. Have you ever touched a 75 watt lightbulb? They're hot as hell. If your human does nothing all day, s/he will be like an inefficient incandescent lightbulb on for a full 24 hours. Maybe that seems low but electricity is relatively cheap these days. To use another classic physics experiment, 1700kcal=7000kJ=a lot of ice turned into steam.
No wonder my father always made me turn the light off in the bathroom...