A gallon of propane weighs approximately 4.2 lbs. And it takes about 92,000 BTUs/hr to burn one gallon of propane.For the sake of calculations, let’s round this figure up to 100,000 BTU/hr. Therefore, if your heater offers 20, 000 BTU/hour, we can determine that your unit will consume propane at a rate of about ⅕ gallons per hour. Thus, a gallon of propane will power your 20,000 BTU propane heater fully for about five hours.