Generator Fuel Consumption Calculator

How much fuel a generator uses depends mostly on two things: how big it is and how hard it is working. Enter your generator’s rated output, the share of it you will actually use and your fuel type. The calculator estimates fuel per hour, total fuel for your outage and what it will cost.

Fuel & running cost

Generator Fuel Consumption Calculator

Estimate how much fuel your generator burns per hour at your expected load, and what it costs to run.

Portable engines use more fuel per kWh than home standby and diesel units.
kW
Use running watts ÷ 1,000. A 7,500 W generator is 7.5 kW.
%
Typical home backup runs at 30–60% of rated output.
$/gal
Example price only. Enter what you pay locally.
hours
For example, 24 hours for a one-day outage.
FUEL ESTIMATE
Estimated fuel use
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Estimates use typical published consumption curves and can differ from your model by 20% or more. Your owner's manual figure at the same load is always more accurate.

How the estimate works

Generator engines burn a fixed amount of fuel just to keep turning, plus an amount that rises with the electrical load. Engineers call this a Willans line, and it fits published consumption charts well:

Fuel per hour = rated kW × (c₀ + c₁ × load fraction)

The coefficients differ by engine type. We fitted them to manufacturers’ published fuel-use figures, then checked them against independent spec sheets such as a 22 kW home standby unit (about 2.4 gal/hr of propane at half load and about 330 ft³/hr of natural gas at full load) and 60 kW diesel charts.

Generator and fuelc₀ (per kW)c₁ (per kW)Unit
Gasoline, portable or inverter0.0300.130gal/hr
Propane, portable or dual fuel0.03740.162gal/hr
Propane, home standby0.04370.1257gal/hr
Natural gas, home standby3.8811.17ft³/hr
Diesel0.0120.065gal/hr

Propane is also shown in pounds (4.24 lb per gallon), and natural gas in therms (about 1,037 BTU per cubic foot, or 0.01037 therms).

Worked example

A 7,500 W (7.5 kW) gasoline portable at 50% load:

  • 7.5 × (0.030 + 0.130 × 0.5) = 0.71 gal/hr
  • For a 24-hour outage: 0.71 × 24 = 17.1 gallons
  • At $3.20 per gallon: $2.28 per hour, or about $55 per day

Typical fuel consumption by size

Estimates at 50% load from the same model. Use them as a quick reference; your owner’s manual is the best source for your exact unit.

GeneratorFuelAt 50% loadPer 24 hours
2,000 W inverterGasoline0.19 gal/hr4.6 gal
3,500 W portableGasoline0.33 gal/hr8.0 gal
7,500 W portableGasoline0.71 gal/hr17.1 gal
7,500 W portablePropane0.89 gal/hr (3.8 lb)21.3 gal
12 kW home standbyNatural gas114 ft³/hr2,726 ft³ (28 therms)
22 kW home standbyPropane2.34 gal/hr56 gal
20 kW dieselDiesel0.89 gal/hr21.4 gal

Ways to use less fuel

  • Right-size the generator. A generator idling at 20% load burns far more fuel per kWh than one working at 60%. See the “cost per kWh produced” figure above.
  • Use eco or idle mode on inverter generators, which lets the engine slow down at light loads.
  • Run big loads together, then shut down. For example, cool the refrigerator and freezer for a few hours, then switch off; a full freezer stays frozen for about 48 hours if kept closed.
  • Keep it maintained. A clogged air filter or old spark plug increases consumption. See our oil change guide.

Frequently asked questions

Does propane use more fuel than gasoline?

Yes, by volume. A gallon of propane holds about 91,500 BTU versus roughly 114,000 for gasoline, so a dual fuel generator burns about 25% more gallons on propane. It also produces about 10% less power on propane.

How long will a 20 lb propane tank last?

A standard 20 lb tank holds about 4.7 gallons. A 3,500 W dual fuel generator at 25% load uses about 1.2 lb per hour, so you get roughly 15 to 17 hours. At half load it uses about 1.8 lb per hour, so expect around 11 hours. Run your numbers in the runtime calculator.

Why is my generator using more fuel than the estimate?

Real consumption varies with engine design, altitude, temperature, fuel quality and maintenance. Motor loads that cycle on and off also raise the average load. If your manual lists consumption at your load, trust that figure over the estimate.

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