Fuel choice looks like a detail until the third day of an outage, when it turns out to be the whole story. Each option trades away something different, and the right answer depends less on the fuel itself than on what your particular emergency does to its supply chain.
Gasoline
The default for portable generators. Cheap hardware, available everywhere, high energy density, and an engine that starts easily in mild weather.
The problem is storage. Pump gasoline begins to oxidise within a few weeks and the light fractions that make it easy to start evaporate first. Ethanol-blended fuel adds a second issue: ethanol is hygroscopic, it pulls moisture out of the air, and once enough water accumulates the mixture separates into layers. The ethanol-water layer settles at the bottom of the tank, which is exactly where the fuel pickup is. That is the single most common reason a generator that ran fine last year will not start this year.
Practical rules: use ethanol-free fuel if you can get it, add a stabiliser at the moment of purchase rather than months later, rotate stored fuel into your car every few months, and either run the carburettor dry before storage or keep the tank full to limit condensation. Also worth knowing: storing enough gasoline for a three-day outage means storing 15 to 25 gallons, which is a meaningful fire load in a garage and is regulated in many jurisdictions.
Propane (LP)
Propane's headline advantage is that it does not go stale. A sealed tank stored properly is as good in ten years as the day it was filled, which makes it the natural choice for equipment that might sit unused for years.
It burns cleaner than gasoline, which means less carbon in the engine and less varnish in the fuel system, and there is nothing to drain before storage. Tanks are exchangeable at any hardware store, and a 100 pound cylinder or a residential tank gives real endurance.
The trade-offs: propane carries roughly 10 percent less energy per unit than gasoline, so the same generator produces slightly lower rated output on LP. And vaporisation slows badly in the cold. Below roughly minus 20 degrees Fahrenheit propane will not build enough vapour pressure to feed an engine, and long before that a small 20 pound cylinder can frost over and starve under sustained draw. Cold-weather propane operation wants a large tank, not several small ones.
Natural gas
The standard for standby generators, because it arrives by pipe and needs no storage at all. Runtime is effectively unlimited, there is nothing to refuel at 3 a.m., and it is the cleanest-burning of the four.
The trade-offs: the lowest energy density of the group, so a natural gas generator is derated compared with the same engine on propane or gasoline. The gas line must be sized for the generator's peak demand, which is a real calculation and sometimes means upgrading the meter and the piping, a line item that surprises people in installation quotes. And the supply depends on the utility. In most storms and blackouts the gas keeps flowing; in an earthquake it may be shut off by design, which is why gas-fired standby is a weaker choice on the west coast than in the midwest.
Diesel
Uncommon in residential sizes, dominant above about 20 kW and in commercial and towable units. Diesel engines are built for sustained load and long service life, and diesel fuel is far less volatile than gasoline, which makes bulk storage safer.
The trade-offs: diesel does degrade in storage, primarily through microbial growth in the water layer at the bottom of the tank, so stored fuel needs a biocide and periodic polishing. Cold weather demands winter blends or additives to prevent gelling. And a diesel engine run at very light load for long periods suffers wet stacking, where unburned fuel accumulates in the exhaust. Diesel wants to be worked.
Dual fuel and tri-fuel
Many portables now ship able to run on both gasoline and propane, and conversion kits add natural gas. This is genuinely useful: you keep a modest amount of gasoline for maximum output and quick starts, and lean on propane for long-term storage and clean shutdowns. During a regional emergency, the fuel that has run out is rarely the same fuel everywhere, and being able to burn either one is worth the small premium.
How much fuel do you actually need?
Start from consumption at your expected load, not the spec sheet's best case. A rough planning figure for a mid-size portable at half load is 0.5 to 0.75 gallons of gasoline per hour, or about 1 to 1.5 pounds of propane. For a three-day outage running 12 hours a day, that is 18 to 27 gallons of gasoline, or two to three 40 pound propane cylinders. Write your own number down before storm season rather than discovering it on day two, when the stations without power cannot pump anyway.
Emissions and the rules that shape the market
Small engines are regulated. The EPA sets emission standards for nonroad engines including portable generators, and California adds its own stricter requirements, which is why some models are sold as "49-state" units. If you live in California or plan to move equipment there, check compliance before buying rather than after. For context on where household energy comes from in the first place, the EIA's electricity explainer is a clear, non-commercial primer.
Quick recommendation
- Occasional use, tight budget: dual-fuel portable. Store propane, keep a little stabilised gasoline.
- Sits unused for years at a time: propane, no question. Nothing to go stale.
- Whole-house standby with reliable gas service: natural gas.
- Rural, large loads, or gas service that is not a given: propane standby with a large owned tank.
- No fuel storage possible at all: that is the case for a battery system, indoors, recharged from solar or the grid.