Gas vs Propane vs Dual Fuel Generators
Updated July 2026
Gasoline packs about 25% more energy per gallon (114,000 BTU vs propane's 91,500) and is everywhere; propane stores forever and starts clean after two years on a shelf. Dual-fuel units take both for a $50 to $150 premium, and for storm-prep buyers that premium is the easiest yes on this site. The tradeoffs, with numbers.
What the energy gap costs you
Run the same unit on propane and you give up roughly 10% of rated output (a "dual fuel 3,500W" is typically 3,500 on gas, 3,150 on propane) and about 20% of runtime per gallon. Our runtime calculator carries the exact factor. In practice this means one extra cylinder swap per day of outage, which matters less than the next section.
Storage is where propane wins outright
Gasoline is a perishable. Stabilized and sealed, call it 12 months before it starts varnishing carburetors; untreated, half that, and the number one reason storm generators fail to start is last hurricane season's fuel. Propane is inert in the cylinder for decades, the cylinder is its own containment (no garage full of jerry cans off-gassing), and a grill tank you already own is 4.7 gallons of it. For equipment that must start after two idle years, propane is the correct engineering answer.
Cold weather flips a different way
Propane cylinders lose vapor pressure as they chill: below about 20°F a 20 lb tank struggles to feed a big engine at full load, and below zero it may not feed it at all (the 100 lb cylinder's larger surface area helps). Gasoline doesn't care until temperatures no generator owner is outside in. Ice-storm country runs gas in January and propane the other nine months, which is the dual-fuel argument compressing itself into one sentence.
Natural gas, the third fuel
Standby installs mostly drink natural gas: unlimited runtime, no storage, no refueling in the rain. The costs are another few percent of output versus propane and dependence on gas infrastructure that big earthquakes (though rarely storms) can interrupt. Tri-fuel conversion kits for portables exist and are popular in hurricane country, where day-nine refueling queues make an unlimited fuel pipe worth an afternoon of carburetor work.
The buying rule
Regular user (job sites, camping, RV): straight gasoline, you'll never store it long enough to go stale, and you want the full rated output. Storm-prep buyer whose generator lives in a shed 51 weeks a year: dual fuel, stored propane, one fresh can of gas when the forecast turns. The premium buys you the fuel that's available that week, which in a real multi-day outage is not a hypothetical.
Questions people ask
How much power do I lose running propane?
About 10% off rated output, from propane’s lower energy density per unit of carbureted volume. A dual-fuel unit rated 4,375/3,500W on gasoline typically books 3,960/3,150W on propane. Size with the propane numbers if propane is your storm plan; our calculators size on gasoline ratings, so mentally shift one class up for propane-primary use.
How long does a 20 lb propane tank run a generator?
A 3,500W-class unit at half load runs roughly 10 to 11 hours on one 20 lb cylinder (4.7 gallons at about 80% of gasoline’s energy). Two grill tanks cover a weekend outage with rationing. The runtime calculator does the math for any unit size and load.
Is stale gasoline really that big a deal?
It is the most common reason a storm generator fails to start, ahead of dead batteries and mice. Ethanol-blend gas absorbs water and varnishes in months; the fix costs a carburetor cleaning at best. If a generator sits more than six months between uses, run it dry after each use, store it fuelless, and let propane or fresh-bought gas be the storm plan.