I thought it’d be worse:

1 btu raises 1 lb water 1 deg. F :)

Need to go from 40 —> 212F = 172 BTU

970 btu to turn 1 lb water to steam (latent heat)

Total Btu / pound to boil off 1 lb water from 40deg F = 1142 Btu

320 oz sap to get to 8 oz syrup, need to boil off 312oz.

312 oz sap = 19.5lbs * 1142btu / lb = 22269 Btu

Assume 32% efficiency propane burner —> pot. 22269 btu * 1/.32 efficiency = 69590.6 Btu propane req’d

69590.6 Btu * 1 gal / 91500btu = 0.76 gal propane

I pay $2.60 / gallon of propane that goes to my stove. Ends up being ~$1.98 for an 8 oz bottle.