Propane BTU content

Higher and lower heating value — and which one to use

Every fuel has two energy figures. The higher heating value counts the heat you would recover if the water vapour in the exhaust condensed; the lower heating value does not. An engine sends its exhaust out hot and never recovers that latent heat, so the lower heating value is the physically relevant number for a generator — while the higher heating value is what the propane industry quotes and bills against. For propane the two are 91,500 and about 84,300 BTU per gallon, roughly an 8% gap.

The same split applies elsewhere. Gasoline runs about 120,000 BTU/gal on a higher-heating-value basis and about 112,000 for a typical E10 blend on a lower-heating-value basis; the widely quoted 114,000 sits between them and is a perfectly good planning figure. What matters is comparing like with like — pairing an HHV propane number against an LHV gasoline number quietly tilts the comparison by several percent.

Neither figure tells you what you get out of the machine. A small air-cooled generator turns roughly 15–22% of the fuel’s energy into electricity at sensible loads, and considerably less at very light load. A gallon of propane holds about 26.8 kWh of chemical energy (91,500 ÷ 3,412 BTU per kWh) and typically yields 4–6 kWh at the outlets. Larger liquid-cooled standby sets sit at the top of that band; a 2 kW inverter idling to keep a router alive sits well below it.

Turning BTU into runtime

There are two routes. If the manufacturer publishes gallons per hour at a load fraction, divide the tank’s usable gallons by it. If it publishes only BTU per hour — common on standby spec sheets, where propane and natural-gas ratings are both given in BTU/hr — divide by 91,500 for gallons per hour of propane, or by 1,030 for cubic feet per hour of natural gas.

Worked across sizes: a 3,500–4,000 W portable at half load burns roughly 0.4–0.6 gal/hr of propane, so one 20 lb cylinder is an eight- to eleven-hour night. A 7,000–9,500 W unit at half load burns roughly 0.8–1.1 gal/hr — about five hours from the same cylinder, which is why people running larger portables on propane end up with a rack of cylinders or a 100 lb tank. A 22 kW air-cooled standby burns roughly 2.1–2.2 gal/hr at half load and 3.5–3.7 at full, so a 500-gallon tank at its 80% fill runs about seven to eight days at half load and about four and a half at full.

Three corrections apply to the arithmetic. Cylinders vaporise more slowly when cold and nearly empty, so the last fifth of a cylinder may not sustain full draw in winter. Fixed tanks are filled to 80% to leave room for thermal expansion, so a “500-gallon” tank is a 400-gallon tank. And the swap cylinder from a supermarket cage is normally filled to 15 lb rather than 20 — about 3.5 gallons and 320,000 BTU, a quarter less than the tank’s nominal capacity, for the same price as a full one.

Comparing fuels on cost rather than on BTU

BTU per gallon is a conversion factor, not an answer. Cost per million BTU is the comparable number: divide the price per gallon by 91,500 for propane, by about 114,000 for gasoline, and the price per therm by 100,000 for natural gas, then scale. At $3.00 per gallon of propane, $3.50 per gallon of gasoline, and $1.20 per therm, that works out to roughly $32.80, $30.70, and $12.00 per million BTU. Natural gas is not marginally cheaper — it is around a third the price of either liquid fuel.

Do not compound that with the output derating. Propane produces about 10% less peak output than gasoline in the same engine and natural gas about 20–30% less, but that is a ceiling on maximum power, not a fuel-efficiency penalty: at the same kilowatt output, the engine consumes roughly the same energy on either fuel, give or take a couple of percentage points. The derating changes which generator you have to buy, not what a kilowatt-hour costs once you own it.

For propane specifically, how you buy it swamps everything else. Exchange cages are the most expensive route twice over — a partial fill at a premium price. Refilling your own cylinders at a propane dealer is meaningfully cheaper per gallon; bulk delivery into a fixed 250 or 500 gallon tank is cheaper again and often on a seasonal contract. Between the cheapest and dearest way to buy the same gallon the price can differ by a factor of two, which is a far bigger lever than any efficiency argument.

The units you will meet on a bill

Propane is sold by the gallon in the US and by the pound when cylinders are exchanged, which is why the same 20 lb cylinder gets described as 4.7 gallons in one place and 430,000 BTU in another. Convert with 91,500 BTU per gallon and roughly 4.2 pounds per gallon and the three descriptions reconcile.

Natural gas is billed in therms (100,000 BTU) or in CCF, meaning 100 cubic feet. At a typical 1,030 BTU/ft³ heat content, one CCF is about 1.03 therms; MCF, 1,000 cubic feet, is about 10.3 therms. Utilities print the exact heat-content factor on the bill because gas composition varies slightly by region and season, so use their number rather than 1,030 when you are checking a bill.