Most portable generators run on regular unleaded gasoline with an octane rating of 87 or higher. That’s the same fuel you put in your car. If you have a dual-fuel or tri-fuel generator, you can also use propane or natural gas, but standard gasoline is the default for the vast majority of portable units sold today.
The type of fuel matters less than the quality and freshness of that fuel. Old gasoline, high-ethanol blends, and improper storage cause more generator problems than almost anything else.
Regular Unleaded Gasoline: The Standard Choice
Regular unleaded gasoline (87 octane) is what generator manufacturers design and certify their engines to use. You don’t need premium fuel. Higher octane ratings won’t improve performance or protect the engine in a small generator the way some people assume.
The critical detail is ethanol content. Your generator is designed to handle gasoline containing up to 10% ethanol (labeled E10 at the pump), which is what most gas stations sell. Do not use fuel with more than 10% ethanol. Blends like E15 or E85 cause starting problems, rough running, and long-term damage to fuel lines and carburetor components. If you can find ethanol-free gasoline (sometimes sold as “recreational fuel” or “REC-90”), that’s the ideal choice, especially for fuel you plan to store.
Propane: Cleaner but Less Powerful
Dual-fuel generators let you switch between gasoline and propane (LPG). Propane burns cleaner, produces less carbon buildup in the engine, and stores indefinitely in a sealed tank. Those are real advantages, particularly for a generator that sits idle for months between uses.
The tradeoff is energy output. One gallon of propane contains only about 73% of the energy in one gallon of gasoline. In practical terms, that means your generator produces less wattage on propane and burns through fuel faster to maintain the same load. A generator rated at 3,500 watts on gasoline might only deliver around 3,000 watts on propane. For light loads like a refrigerator, some lights, and phone chargers, that’s fine. For heavy loads like a well pump or central AC, the reduced output could matter.
Natural Gas: For Standby Generators
Whole-house standby generators typically connect directly to your home’s natural gas line. The fuel supply is essentially unlimited during an outage (as long as the gas utility is still operating), so you never have to worry about refueling or storage. Natural gas also burns very cleanly.
These systems require a dedicated gas line sized to deliver enough fuel at the right pressure, typically 3.5 to 7 inches of water column. A licensed installer handles this during setup. You can’t simply hook a portable generator to a natural gas line without a conversion kit and proper plumbing, and doing so incorrectly is dangerous.
How Long Gasoline Lasts in Your Generator
Untreated gasoline starts degrading in as little as 30 days and is generally considered unreliable after 3 to 6 months. As it breaks down, it forms a gummy varnish that clogs the carburetor’s tiny jets and passages. This is the single most common reason generators won’t start when you actually need them.
Fuel stabilizer slows this process significantly. Adding a stabilizer when you fill up can extend gasoline’s usable life to 12 months or more, depending on the product. Popular options include STA-BIL Storage (widely available and inexpensive) and PRI-G (a favorite among long-term preparedness planners). Sea Foam works both as a stabilizer and a cleaner for engines that have already developed minor buildup. Whichever you choose, add it to fresh gasoline, not fuel that’s already been sitting.
If your generator uses ethanol-blended fuel, stabilizer becomes even more important. Ethanol attracts moisture from the air, and over time the water content reaches a tipping point where the ethanol and water separate from the gasoline entirely. This is called phase separation, and once it happens, no stabilizer can fix it. The fuel is ruined.
Storing Fuel Safely at Home
NFPA guidelines limit residential gasoline storage to 25 gallons total, and that fuel should be kept in an unattached garage or shed whenever possible. If your only option is an attached garage, the limit drops to 10 gallons. Never store gasoline in a basement.
Use approved containers, the red plastic or metal cans with DOT or UL ratings. Keep them sealed, away from heat sources and direct sunlight, and off bare concrete (which can cause condensation on the bottom of the container). For most households, two 5-gallon cans with stabilized fuel provide a solid 8 to 12 hours of runtime for a mid-sized portable generator.
High Altitude Changes Fuel Needs
If you live or camp above 2,000 feet elevation, your generator’s fuel-air mixture becomes too rich because the air is thinner. The engine still pushes the same amount of fuel, but there’s less oxygen to burn it. The result is wasted fuel, reduced power, and higher emissions. Engine horsepower drops about 3.5% for every 1,000 feet of elevation gain, even after adjustments.
Some generators have a high-altitude carburetor jet that can be swapped in to lean out the mixture. Honda and other manufacturers publish specific jetting instructions for their models. If you’re running a generator above 5,000 feet regularly, this adjustment is worth making. It won’t fully restore sea-level power, but it improves efficiency and reduces carbon fouling on the spark plug.
Quick Fuel Comparison
- Gasoline: Highest power output per gallon, available everywhere, but degrades in storage and requires stabilizer for long-term readiness.
- Propane: Stores indefinitely, burns cleaner, but delivers roughly 25% less energy per gallon and requires a dual-fuel capable generator.
- Natural gas: Unlimited supply from your utility line, extremely clean burning, but limited to permanently installed standby generators with professional hookup.
For most people buying a portable generator for emergencies or job sites, fresh regular unleaded gasoline with a fuel stabilizer added at fill-up is the simplest and most effective approach. If you want the convenience of long-term storage without worrying about fuel going bad, a dual-fuel model running on propane gives you that flexibility.