A generator’s fuel use depends far more on the load you put on it than on the size of its engine. A 7,500 watt generator running a refrigerator and a few lights burns a fraction of what it burns at full load. This guide gives typical figures, shows how to read the real numbers from a specification sheet, and lists the habits that stretch a tank of gas.
Quick answer
A portable gasoline generator uses roughly 0.15 to 0.3 gallons of fuel for each kilowatt-hour it produces. At half load, that works out to about 0.3 to 0.4 gallons an hour for a 3,500 W unit, 0.6 to 0.75 for a 5,000 W unit, and 0.9 to 1.1 for a 7,500 W unit. Running 24 hours at half load, a mid-size generator uses around 8 to 15 gallons a day.
In this guide
Typical fuel consumption by generator size
The figures below are planning estimates based on how small gasoline engines typically perform. Individual models vary, inverter generators do better at light loads, and consumption rises sharply above 75 percent load.
| Generator size | 25 percent load | 50 percent load | Full load |
|---|---|---|---|
| 2,000 W inverter | 0.1 to 0.15 | 0.15 to 0.25 | 0.3 to 0.4 |
| 3,500 W | 0.2 to 0.3 | 0.3 to 0.4 | 0.6 to 0.8 |
| 5,000 W | 0.3 to 0.4 | 0.6 to 0.75 | 1.0 to 1.3 |
| 7,500 W | 0.5 to 0.6 | 0.9 to 1.1 | 1.4 to 1.8 |
| 10,000 to 12,000 W | 0.6 to 0.8 | 1.2 to 1.6 | 2.0 to 2.5 |
How to get the real number for your generator
Every manufacturer publishes a tank size and a run time at a stated load, usually 25 or 50 percent. Divide one by the other.
For example, a 7,500 W generator with a 6.6 gallon tank that is rated for 8 hours at 50 percent load uses 6.6 ÷ 8, or about 0.8 gallons an hour at that load. Since 50 percent load is 3,750 watts, that is roughly 0.22 gallons per kilowatt-hour, which is typical for an open-frame unit.
Keep in mind that run-time ratings are measured at a steady load. A household load that cycles on and off, with a refrigerator compressor starting every few minutes, usually averages less than you would guess from adding up the nameplates.
Propane and natural gas
Propane contains about a quarter less energy per gallon than gasoline, so a dual-fuel generator burns more gallons of propane for the same output. A standard 20-pound cylinder holds about 4.7 gallons and typically runs a mid-size generator for 8 to 12 hours at half load. For a standby generator, the specification sheet lists consumption in cubic feet of natural gas or gallons of propane per hour at half and full load. As a rough guide, a 20 to 24 kW air-cooled standby unit uses something like 200 to 350 cubic feet of natural gas an hour, or 2.5 to 4 gallons of propane, depending on load. See our guide to dual-fuel generators for more on running on propane.
Diesel
Diesel engines convert fuel to power more efficiently. Diesel generators typically use around 0.07 to 0.1 gallons per kilowatt-hour, so a 10 kW diesel set at half load burns roughly half a gallon an hour. The fuel also stores better than gasoline. Our comparison of diesel vs gas generators covers the trade-offs.
How much fuel to store for an outage
Work from your real load, not the generator’s rating, and decide how many hours a day you will actually run it. Many households run a generator for a few hours at a time: long enough to cool the refrigerator and freezer, run the furnace and charge devices, then shut it down. That approach can cut fuel use by half or more compared with running around the clock.
As an example, a household that runs a 5,000 W generator at roughly half load for 8 hours a day uses about 5 to 6 gallons a day, or 15 to 18 gallons for a three-day outage. Keep that fuel in approved containers, treated with stabilizer, and rotate it into your car every few months. Local fire codes limit how much gasoline you can store at home, so check before stockpiling. Read how long gasoline lasts before you fill the cans.
Ways to use less fuel
- Run only what you need. Electric heaters, water heaters and ovens are the worst offenders. Cook on a camp stove and use a gas water heater’s pilot if you have one.
- Cycle the generator. A closed refrigerator stays cold for about four hours and a full freezer for about two days. You do not have to run it continuously.
- Use the eco or idle mode on an inverter generator so the engine slows under light loads.
- Do not oversize. A large generator at 10 percent load is inefficient. Match the generator to the load.
- Keep it maintained. A clean air filter, fresh spark plug and correct oil all help the engine run efficiently.
- Stagger big loads so the generator is not forced to run near full output.
Frequently asked questions
How long will a generator run on 5 gallons of gas?
At half load, a 5,000 W generator uses about 0.6 to 0.75 gallons an hour, so 5 gallons lasts roughly 7 to 8 hours. A 2,000 W inverter at a light load can stretch 5 gallons to well over a day.
Does a generator use more gas with more things plugged in?
Yes. Fuel use rises with the electrical load. At a quarter load most generators burn roughly half as much as they do at full load, and an inverter generator saves even more at light loads.
Is it cheaper to run a generator on propane or gas?
Usually gasoline, because propane has less energy per gallon. Propane's advantages are that it stores indefinitely and burns cleanly, not that it is cheaper per hour.
