What a BTU is
A British Thermal Unit is the energy required to raise the temperature of 1 pound of water by 1°F. In the US gas industry, BTUs measure how much energy an appliance consumes per hour (BTU/hr) or how much energy a fuel delivers per unit (BTU/gallon for liquid propane).
Propane BTU conversions
- 1 gallon liquid propane = 91,500 BTU
- 1 lb propane = 21,548 BTU (4.24 lb per gallon)
- 1 cubic ft propane vapor = 2,516 BTU
- 1 gallon = ~26.8 kWh equivalent (at 100% conversion; appliance efficiency reduces actual delivered)
Typical residential appliance BTU loads
- Furnace — 60,000 to 100,000 BTU/hr (whole-home heating)
- Tank water heater — 30,000 to 75,000 BTU/hr
- Tankless water heater — 150,000 to 199,000 BTU/hr (peak)
- Range — 50,000 to 80,000 BTU/hr total (cooktop + oven)
- Clothes dryer — 20,000 to 25,000 BTU/hr
- Standby generator (20 kW) — 200,000+ BTU/hr at full load
- Pool heater — 150,000 to 400,000 BTU/hr
Sizing math example
Worked example: an 80,000 BTU/hr furnace running 1 hour consumes 80,000 ÷ 91,500 = 0.87 gallons. Over a moderate winter (~1,500 furnace-run hours), that's ~1,300 gallons just for heating. Add water heater (~200 gal/yr), cooking (~30 gal/yr), dryer (~25 gal/yr) for a whole-home estimate. See calculate your propane usage.
FAQ
Where do I find an appliance's BTU rating?
On the manufacturer's nameplate (typically a metal plate on the back, bottom or inside of the appliance) and in the owner's manual. The number is labelled 'Input BTU' or 'BTU/hr Input'.
Are input BTU and output BTU the same?
No. Input BTU is the fuel consumption; output BTU is the heat actually delivered after combustion losses. A modern 95% AFUE furnace with 100,000 BTU input delivers ~95,000 BTU of heat. Use input BTU for fuel-consumption calculations.