Does a generator run longer on gas or propane?

This question has two honest answers. It depends on whether you mean per gallon or per tank.

Per gallon: gas runs longer

A gallon of gasoline holds more energy than a gallon of propane. The U.S. Department of Energy lists:

Fuel	Energy per gallon
Gasoline	112,114–116,090 Btu
Propane	84,250 Btu

So a gallon of propane has about 73% to 75% of the energy in a gallon of gas. On the same generator at the same load, a gallon of propane won't last as long as a gallon of gas.

Per tank: it depends on the tank sizes

You don't buy propane by the gallon for a generator, though. You hook up a tank, usually a 20 lb one.

How much gas is a 20 lb tank "worth"?

1. Propane weighs about 4.2 lb per gallon, so 20 lb ≈ 4.8 gallons of propane.
2. 4.8 gallons × 84,250 Btu = about 400,000 Btu.
3. 400,000 ÷ about 114,000 Btu per gallon of gas = about 3.5 gallons of gas.

So here's the simple rule:

• If the generator's gas tank holds less than about 3.5 gallons, a full 20 lb propane tank usually runs it longer.
• If the gas tank holds more, a full tank of gas usually runs longer.

What the makers' own numbers say

Here are published run times for the same machines on both fuels. Load % is the share of the generator's rated watts in use.

Model	Gas tank	Run time on a full gas tank	Run time on a 20 lb propane tank
Westinghouse iGen2550DFc	1.1 gal	11 h at 25%; 7 h at 50%	25 h at 25%; 20 h at 50%
Westinghouse iGen4000DFc	1.69 gal	7 h at 25%	17 h at 25%
WEN DF480iX	1.9 gal	7 h at 50%	12 h at 50%
Westinghouse WGen7500DFc	6.6 gal	11 h at 25%	8 h at 25%
Westinghouse WGen9500DFc	6.6 gal	12 h at 25%; 8 h at 50%	7 h at 50%

The rule holds for all five. The three small inverters (1.1 to 1.9 gal tanks) run longer on a 20 lb tank. The two big open-frame models (6.6 gal tanks) run longer on a full tank of gas.

Worked example: is the rule close?

Take the WEN DF480iX at half load.

• On gas: 7 hours ÷ 1.9 gallons = about 3.7 hours per gallon
• A 20 lb tank ≈ 3.5 gallons of gas: 3.7 × 3.5 = about 13 hours
• WEN's own figure: 12 hours

Close. But it doesn't work that neatly for every model. Do the same sum for the WGen9500DFc at 50% and you get about 4 hours, while Westinghouse lists 7. For the five models above, makers' propane figures run from about 30% below to about 65% above this estimate. Use the maker's propane figure when there is one. Use the rule only when there isn't.

Why the per-hour answer isn't the whole story

Propane makes less power. Makers rate their dual fuel models lower on propane, by about 5% to 10%. The WGen9500DFc is rated 9,500 running watts on gas and 8,500 on propane. So "50% load" on propane is a smaller load in watts. That's one reason the propane run time can look better than the energy math suggests.

Swapping tanks is easy. A gas tank needs refilling with the engine off and cool. A propane tank can be swapped for a full one. With two or three 20 lb tanks, run time is mostly a matter of how many tanks you keep. A 30, 40 or 100 lb tank runs about 1.5, 2 or 5 times as long as a 20 lb one. See how long a generator runs on a propane tank.

Gas goes stale; propane doesn't sit in the generator. Makers warn about old gasoline. WEN, for example, says to use gas within 30 days of buying it. A sealed propane tank isn't part of that problem. See dual fuel vs gas generators.

How to stretch either fuel

• Run at a lighter load. Halving the load doesn't double the run time, but it helps a lot. Compare the 25% and 50% figures in the table.
• Use eco or economy mode on an inverter generator at light loads.
• Turn off what you don't need, like a space heater or a second freezer.
• Refuel safely. Engine off and cool, outdoors, no flames. Never refuel a running generator.

For gas-only planning, see how long a generator runs on 5 gallons of gas. The calculator above is set to the WGen9500DFc on a 20 lb tank at 50% load.

Safety: Propane exhaust has carbon monoxide too. Run it outdoors only, 20+ feet from the house, away from windows and doors. Read our carbon monoxide guide.