Wonderful inputs, thank you all. I enjoy the range of detail and assumptions.
So if I use one pot of water per day,, on average, I’m thinking it’s probably a little higher than Bilby and Rousseau calculated due to the low efficiency of the furnance and its air delivery system in this old house.
Enjoyable to think about the alternative heat from friends visiting. Multiply that by a bit higher since most of my friends are menopausal, so the amount of body heat on surface skin is just slightly higher, and tehy tend to wear tank tops in winter at least part of the time, exposing more skin to the air.
So I may only get a nickel or less a day, and save about 32ml of oil, which on a 6-month heating season comes out about 1.5 gallons a year,
BUT, and hear me out, here,
I will avoid the muddy spot in the yard during winter over the septic tank where the household temp water always keeps the ground above freezing.
Then again, there's all that drama to deal with. I think I'll stick with my wall heater and comforter.