About the Generator Fuel Consumption Calculator
This generator fuel consumption calculator estimates how much fuel a gasoline, diesel or propane generator burns per hour, how long a tank will last and what it costs to run. Enter the generator’s rated output in kW, the load you expect as a percentage of that rating, the fuel you have on hand and your fuel price.
It helps homeowners planning for a power outage, campers and RV owners, contractors running tools on site and anyone deciding how much fuel to store. The table shows fuel use at quarter, half, three-quarter and full load, so you can see how much you save by running only essential circuits.
Generators burn fuel even when lightly loaded, so fuel use does not fall in proportion to load. The calculator uses a straight-line fuel curve in which about a quarter of full-load fuel is burned at zero load, with typical full-load rates for each fuel taken from manufacturer data. If you know your generator’s full-load consumption from its spec sheet or manual, switch on the spec-sheet option for a more accurate result. Inverter generators with eco-throttle can use noticeably less fuel at light loads.
With the default inputs, the fuel consumption is 0.68 gal/h. Change any value above to recalculate instantly.
How to use the generator fuel consumption calculator
- 1Choose the fuel your generator burns and enter its rated running output in kW.
- 2Estimate the load as a percentage of that rating.
- 3Optionally enter the full-load fuel consumption from the spec sheet.
- 4Enter the fuel you have, hours of use per day and the fuel price.
- 5Read the gallons per hour, runtime and running cost.
Formula and method
Generator fuel use is close to a straight line between an idle consumption and full-load consumption. This calculator assumes the engine burns about 25% of its full-load fuel with no load, rising linearly to 100% at full load. Full-load consumption is taken from your spec sheet if you enter it, or estimated from typical figures: about 0.135 gallons per kWh of rated output for gasoline, 0.085 for diesel and 0.165 for propane (propane holds less energy per gallon).
Runtime is the fuel available divided by gallons per hour. Electricity per gallon is the power delivered (rated kW × load) divided by fuel per hour, and fuel cost per kWh is the hourly fuel cost divided by power delivered. Real consumption varies with engine condition, altitude, temperature and how steady the load is.
- F_full
- Fuel consumption at 100% load (gal/h)
- Load
- Fraction of rated output being used (0–1)
- 0.25
- Share of full-load fuel burned at zero load
Worked examples
8 kW portable gasoline generator at half load
Full-load use is estimated at 8 × 0.135 = 1.08 gal/h; at 50% load that becomes 1.08 × (0.25 + 0.375) = 0.675 gal/h. A 7.5 gallon tank lasts about 11 hours, and 8 hours a day costs about $18.90 at $3.50 per gallon.
20 kW diesel standby at 75% load
A 20 kW diesel set is estimated to burn 1.7 gal/h at full load and 1.7 × 0.8125 = 1.38 gal/h at 75%. A 50 gallon tank runs it about 36 hours, and continuous operation costs about $133 a day at $4 per gallon.
22 kW propane standby using the spec-sheet figure
With a spec-sheet full-load rate of 3.6 gal/h, half load uses 3.6 × 0.625 = 2.25 gal/h. The 400 gallons in a full 500 gallon tank would run the generator around the clock for about 7.4 days.
Frequently asked questions
How much gas does a generator use per hour?+
A typical portable gasoline generator burns roughly 0.1–0.14 gallons per hour for each kW of rated output at full load. An 8 kW unit uses about 1.1 gal/h flat out and around 0.7 gal/h at half load.
How long will a generator run on a tank of fuel?+
Divide the tank size by fuel use per hour at your load. A 7.5 gallon tank on an 8 kW generator at half load lasts about 11 hours; at quarter load it lasts roughly 16 hours.
Is diesel or propane cheaper to run a generator on?+
Diesel engines are the most efficient and diesel holds more energy per gallon, so they use the fewest gallons per kWh. Propane generators burn more gallons, but propane stores indefinitely and is often bought in bulk at a lower price.
Why doesn’t fuel use halve when the load halves?+
An engine needs fuel just to turn at governed speed and overcome friction, even with no electrical load. That idle consumption is roughly a quarter of full-load fuel for many generators, so efficiency is best between about 50% and 80% load.
How much propane does a whole-house generator use?+
A 20–22 kW air-cooled standby generator typically uses around 2–3.6 gallons of propane per hour depending on load. A full 500 gallon tank (about 400 gallons) lasts roughly 5–8 days of continuous running.