Generator Fuel Efficiency: Gasoline vs Propane vs Diesel

What affects generator fuel consumption

Fuel consumption is not a fixed number. It moves with the electrical load you place on the generator, the speed of the engine, the state of maintenance, and the design of the unit. A generator running near its rated maximum load burns far more fuel than one running lightly for the same clock time. This is why manufacturers publish run times at a fraction of maximum load, often at half load, and why you should compare those figures rather than marketing claims. See how to read a generator spec sheet for a guide to load-based ratings.

Engine speed also matters. Many portable generators run at a fixed speed to produce standard household power, whether you are charging a phone or running a resistive heater. Inverter generators can slow the engine to match the load and therefore tend to use less fuel in mixed real-world use. For the same reason, an inverter model is often a better value if you expect to run at partial load for long periods; see best inverter generators.

To put load in perspective, the average U.S. household consumes about 10,500 kilowatthours of electricity per year. A generator used during an outage typically powers only part of that load, so the hourly fuel burn is much lower than a continuous maximum rating. Keep in mind that carbon monoxide is produced by any fuel and is deadly; always run generators outdoors more than 20 feet from windows, doors, and vents, and install a CO alarm. The U.S. Consumer Product Safety Commission has also proposed a safety standard for portable generators that includes CO shutoff systems.

Gasoline generators: the everyday option

Gasoline remains the most convenient fuel for portable generators in most of the United States. Nearly every gas station sells it, and the same fuel that runs your car often powers your generator. For an occasional storm outage, a gasoline-powered generator is simple to understand and easy to refuel. However, gasoline is more volatile than diesel and has a shorter useful storage life, especially when it contains ethanol, which absorbs moisture over time. See generator fuel stabilizer and storage life and generator ethanol-free fuel.

Among portable models, gasoline is also the default choice for many open-frame and inverter generators in the sizes that cover common home backup and camping needs. If you plan to run a few appliances for a few hours at a time, a gasoline generator is a low-friction option, and the smaller inverter classes tend to be gasoline-only.

Propane generators: storage friendly

Propane, or LPG, is stored as a pressurized liquid in a tank and vaporizes to gas when the generator draws fuel. A propane tank does not degrade the way gasoline with ethanol can, and a sealed tank can hold propane for years, which is attractive for emergency preparedness. The main tradeoff is energy content per volume: propane contains less energy per gallon than gasoline or diesel, so a generator tends to consume more fuel by volume to produce the same power. See propane vs gasoline portable generators for the practical effect on run time.

Because the fuel is delivered as a vapor, a propane generator must have its fuel pressure regulated, and the tank must be sized to supply enough vapor for the engine at full load. Many users therefore pick a dual fuel generator that runs on either gasoline or propane, using gasoline when they want maximum output and propane for long storage or emergency use. For an installed fuel supply, some home standby generators run on propane as well.

Diesel generators: fuel dense and steady

Diesel produces more energy per gallon than gasoline, so a diesel generator generally burns less fuel by volume to make the same amount of electricity. That makes diesel the most fuel efficient of the three common fuel types in the literal sense, especially under a steady load. Diesel also stores well and is less volatile than gasoline. The tradeoffs are practical: diesel portable generators are less common in the small inverter and open-frame classes aimed at home use, and they appear more often as large portable machines or standby units.

If you need a high-output unit for a home or small business, a diesel generator is worth considering for fuel economy. Most buyers, however, will find more portable options among gasoline and propane models in the sizes they need.

Run time and fuel tank size

Run time is determined by three things on a spec sheet: fuel tank size, fuel consumption rate at a given load, and the generator's rated output. The most useful specification is the run time at half load, because most users do not run continuously at maximum. Compare run times among models only when the load basis is the same. See generator runtime at half load before you compare.

Also pay attention to fuel tank size. A large tank can offset lower fuel efficiency: a propane generator with a big tank may run longer than a gasoline generator with a small one, even though the propane unit uses more fuel per hour. For long outages, external tank hookups for propane and natural gas can extend run time beyond an onboard tank. See generator fuel tank size and run time.

What to pick for your use

Start with your load: total wattage, how long you expect to run, and whether that load is constant or swings. Then match the fuel to your storage and refueling situation. For most people, a gasoline or dual fuel portable unit in the common sizes covers the typical uses, while a diesel or large standby unit fits continuous, high-load needs. The short choices in the table below give next steps.

What to pick for your use

Questions

Which fuel is most efficient for a generator?

At the same load, diesel contains more energy per gallon than gasoline, and gasoline contains more than propane. So a diesel generator uses the least fuel by volume to make the same power. The difference matters most on long, steady runs; for short outages, fuel availability often matters more.

Does propane burn longer than gasoline per tank?

No, the opposite is true at the same tank size. Because propane has less energy per gallon than gasoline, a propane generator will use more fuel by volume for the same work, so the run time between refills is shorter. A larger external tank can change that comparison, though; tank size is what you actually drain.

Can I convert a gasoline generator to run on propane?

Conversion kits exist for many small engines, and dual fuel and tri fuel generators come from the factory with both fuel systems. A conversion only makes sense if the engine and fuel delivery are compatible, so check the maker's guidance before starting.

Do diesel generators need more maintenance than gasoline ones?

Diesel engines are generally robust and can run long under load, but they have their own maintenance needs, including fuel filters and cold-start assistance. The schedule matters more than the fuel type; follow the maker's intervals for oil, filters, and general service.

Which fuel gives the longest run time between refills?

For the same fuel tank size, diesel gives more running time than gasoline, and gasoline gives more than propane, because of the energy content difference. In practice, tank size and load matter more, and a propane unit with a large external tank can outlast a diesel with a small onboard tank.