Dual-fuel and tri-fuel generators explained (and the derating math)
Updated · Reviewed against the current BackupAtlas dataset
Short answer: a dual-fuel generator runs on gasoline or propane; a tri-fuel generator adds natural gas. Switching is a selector switch and, on tri-fuel units, a regulator or quick-connect fitting — not a rebuild. The trade-off is output: propane typically derates a generator's gasoline rating by roughly 10%, and natural gas by roughly 20–25%, because both fuels carry less energy per unit of fuel-air mixture than gasoline vapor.
Why the derating happens
An engine's power comes from burning a fuel-air mixture in the cylinder; what varies is how much energy per unit of mixture is available. Gasoline vaporizes into a dense, energy-rich charge. Propane and natural gas are already gaseous, so they displace more of the cylinder's air with fuel and burn a marginally less energy-dense mixture — the engine makes the same RPM but less torque, hence less electrical output. Natural gas has a lower BTU content than propane, which is why it derates further; see the propane BTU content glossary entry for the exact figures.
How switching actually works
On a dual-fuel portable or inverter unit, a selector switch on the control panel picks gasoline or propane; the carburetor (or a dedicated propane regulator) meters the chosen fuel, and a propane hose connects to a standard cylinder. There is no engine rebuild or tool required to switch fuels — it is intended to be changed in the field, including mid- outage.
Tri-fuel conversions add natural gas as a third option, most often on home-standby units that are permanently plumbed to the gas line, but also on some larger portables via an aftermarket or factory quick-connect kit. A natural-gas hookup on a portable unit still requires a plumber or gas fitter to install a properly sized quick-connect tee — it is not a DIY gas-line job.
The real trade-offs
- Gasoline is the most energy-dense and gives the highest rated output, but it stales in a matter of months without a stabilizer, gums carburetors after sitting, and is the first thing to disappear from stations during a regional outage.
- Propane stores indefinitely in a sealed cylinder, burns cleaner (no carburetor varnish from stale fuel), and is often available even when gas stations lose power — at roughly 10% less output than the gasoline rating. A standard 20 lb grill cylinder holds about 4.6 gallons.
- Natural gas draws from the home's gas meter for effectively unlimited runtime with zero fuel logistics — no cans, no deliveries, nothing to stabilize — at the cost of the deepest derating (typically 20–25% below the gasoline rating) and a meter/line- capacity check alongside the furnace and water heater.
Sizing against the fuel you'll actually run on
The headline wattage on a dual-fuel or tri-fuel generator's nameplate is almost always its gasoline rating. If propane or natural gas is your real emergency plan — because that's the fuel you can reliably access during an outage — size against the derated figure, not the gasoline number. The sizing calculator and the sizing guide both fold in the recommended 20% headroom, which absorbs most of a typical derating on its own, but check the specific model's published propane and natural-gas ratings before assuming it covers your load on either fuel.
In the BackupAtlas database, dual-fuel generators run on gasoline and propane, tri-fuel generators add natural gas, and standby units capable of natural gas are listed under natural-gas generators. For example, the Champion 7500W Dual Fuel (100891) publishes both a 7,500 W gasoline running rating and a 6,750 W propane running rating — a 10% derating in practice — and several best dual-fuel portable picks show the same pattern: each model's own page carries its full per-fuel numbers rather than a single headline figure.