Dual fuel vs gas generator: is dual fuel worth it?

A dual fuel generator has one engine that runs on either gasoline or propane. You flip a selector to choose. A gas-only generator runs on gasoline and nothing else. The real questions are how much power you give up on propane, and whether easier fuel storage is worth it.

We compared these from the makers' published specs; we haven't tested them.

The short version

	Dual fuel (on propane)	Gas only
Power	About 5–10% lower than its gas rating	Full rated power
Fuel storage	Propane stays sealed in its cylinder	Gas goes stale; makers say use it fresh or add stabilizer
Refueling	Swap a tank, no pouring	Pour gas, with the engine off and cool
Cold weather	Propane can vaporize slowly in a very cold tank	Starts on gas as usual (choke)
Energy per gallon	84,250 Btu	112,114–116,090 Btu
Complexity	Extra regulator, hose and selector	Simpler
CO danger	Same: exhaust is deadly on either fuel	Same

How much power do you lose on propane?

This is the propane "derate." The honest way to measure it is from each maker's own two ratings for the same machine:

Model	Running watts on gas	Running watts on propane	Drop
Westinghouse iGen2550DFc	1,900 W	1,700 W	about 11%
Westinghouse iGen4000DFc	3,300 W	2,970 W	10%
WEN DF480iX	4,000 W	3,600 W	10%
Westinghouse WGen7500DFc	7,500 W	6,750 W	10%
Westinghouse WGen9500DFc	9,500 W	8,500 W	about 11%
DuroMax XP13000EH	10,500 W	10,000 W	about 5%

Drop = (gas watts − propane watts) ÷ gas watts.

So plan on losing about a tenth of your power on propane. If you'll mostly use propane, size for the propane number.

Worked example

You need 3,200 running watts for a fridge, a sump pump, lights and a furnace fan.

• On gas, a 3,300 W model covers it, barely.
• On propane, the same model gives 2,970 W. That's 230 W short.

So you'd step up to a model rated at least 3,200 W on propane. With some headroom, that's around 3,600 W on propane.

Why propane gives less power

A gallon of propane holds less energy than a gallon of gasoline. The U.S. Department of Energy lists propane at 84,250 Btu per gallon and gasoline at 112,114 to 116,090 Btu. That's roughly 73% to 75% as much energy, gallon for gallon. Makers' propane ratings are lower for the same engine.

It also means a gallon of propane runs a generator for less time than a gallon of gas. Propane is usually bought by the tank, though. See generator run time on propane vs gas for tank-to-tank numbers.

Storage: the main reason people buy dual fuel

Gasoline doesn't keep well. Makers say so plainly:

• WEN says to use gas within 30 days of buying it, and never to store the generator more than 2 months with fuel in the tank.
• Champion says gas in the tank lasts up to a year with a fuel stabilizer, but gas left in the carburetor can clog it within two weeks.
• Westinghouse says to add stabilizer and to drain the carburetor before storing for two to six months.

Stale gas is a top reason a generator won't start after sitting. See generator won't start after sitting.

Makers' stale-fuel warnings are about gasoline. A propane cylinder stays sealed and outside the generator. That's why many people keep a couple of full tanks for storm season. Makers say to store cylinders away from sparks, flames, pilot lights and heat, and never near a furnace or water heater.

Cold weather: where propane can struggle

Propane has to boil off into gas inside the tank. Champion's dual fuel manual explains that when a tank gets very cold, propane vaporizes more slowly and may not supply enough fuel. Frost on the tank and regulator is common and normally isn't a problem, unless the generator also starts to lose power.

Westinghouse rates the iGen4000DFc for continuous use from 23°F to 104°F.

If you live where winters are hard:

• Champion notes that the container's size and how much fuel you're drawing both affect frost. A heavy load on a small, cold tank is the worst case.
• Keep gas on hand as a backup if your generator is dual fuel.
• Use your manual's cold-start choke steps for propane. See generator won't start on propane.

Which is right for you?

Dual fuel suits you if:

• the generator will sit unused for months between outages
• you'd rather swap a tank than pour gas
• you want a second fuel in case gas stations are closed after a storm

Gas only suits you if:

• you use the generator often, so the gas stays fresh
• you want full rated power at the lowest weight and fewest parts
• you live where propane tanks are hard to refill

Both need the same safety habits. Propane exhaust has carbon monoxide too. Pick a model with a CO shut-off sensor, whatever the fuel.

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