Propane BTU content
Propane carries about 91,500 BTU per gallon — the number that converts a generator’s gallons-per-hour appetite into runtime from a given tank. A 20 lb grill cylinder holds about 4.6 gallons (~420,000 BTU); gasoline carries about 114,000 BTU per gallon.
The BTU gap between propane and gasoline (roughly 20% per gallon) is one reason dual-fuel generators burn propane faster in gallons and produce less peak power on it.
For planning: a generator consuming 1.5 gallons of propane per hour at half load empties a 20 lb cylinder in about three hours, a 100 lb cylinder (~23.6 gal) in about 16 hours, and a 500-gallon home tank (filled to 80%) in roughly 11 days. Natural gas, by comparison, is metered in cubic feet — about 1,030 BTU/ft³, 97 ft³ ≈ 1 therm.