Generator Fuel Consumption Calculator
This generator fuel consumption calculator estimates how much diesel, gasoline, propane, or natural gas a generator burns at a given load. Enter the generator rating, the load as a percentage, the fuel, and the runtime, and the generator fuel consumption calculator returns the fuel used per hour, the total for the run, and how long a full tank lasts.
Results
The Generator Fuel Consumption Calculator is part of our AC & Three Phase collection. Browse more ac & three phase to size, convert, and check your work alongside this tool.
What the generator fuel consumption calculator does
A generator burns some fuel just to keep turning, plus fuel in proportion to the power it delivers. That is why a generator at quarter load uses far more than a quarter of its full-load fuel. The generator fuel consumption calculator uses the standard linear fuel curve for diesel sets, then adjusts for the energy content and efficiency of gasoline, propane, and natural gas engines.
Generator fuel consumption formula
For diesel the calculator uses the linear fuel curve common in energy modelling:
The first term is the no-load fuel; the second grows with output. For spark-ignition fuels, the calculator converts the diesel fuel energy (38.6 MJ/L) using about 25% more energy input, then divides by the fuel energy: gasoline 34.2 MJ/L, propane 25.3 MJ/L, and natural gas about 1,000 BTU per cubic foot. Maker data for your exact model always takes priority.
Generator fuel consumption calculator example
A 20 kW diesel generator at 50% load:
Over 24 hours that is about 26.3 gallons (99 L). The same 20 kW set running on propane at 50% load uses about 2.09 gal/h, and on natural gas about 189 cubic feet per hour.
Generator run time calculator: how long a tank lasts
Runtime is the usable fuel divided by the fuel use per hour. Enter the tank size and the calculator shows it directly:
- Diesel generator run time: a 20 kW set at 50% load with a 100 gallon tank runs about 91 hours.
- Propane generator run time: the same 20 kW set on a 500 gallon propane tank, filled to the usual 80% (400 gallons), runs about 191 hours at 50% load.
- Gas generator run time: a 7.5 kW portable gasoline generator at half load burns about 0.58 gal/h, so a 7 gallon tank lasts about 12 hours.
Plan on a reserve of about 20% so the generator never runs dry, which can draw air into a diesel fuel system.
How to use the generator fuel calculator
Enter the generator rating in kW from its nameplate and the expected load as a percentage of that rating. Choose the fuel. Add the hours you expect to run to get the total, and for liquid fuels add the tank size in gallons to see how long a full tank lasts. Press Calculate.
Reading your generator fuel usage
Fuel per kWh is lowest near 75 to 100% load and climbs steeply at light loads. A 20 kW diesel set uses about 0.41 L per kWh at half load but 0.33 L per kWh at full load. If your generator spends most of its time lightly loaded, a smaller unit may save fuel. Cold weather, altitude, and wear all raise consumption.
When to use a generator runtime calculator
- Planning fuel for a power outage or storm
- Sizing a fuel tank for a standby generator
- Estimating fuel cost for a job site
- Comparing diesel and propane running costs
- Checking delivery schedules for remote sites
Common generator fuel calculator mistakes
- Scaling fuel linearly from full load. Light loads use proportionally more.
- Using kVA as kW. Convert with the power factor.
- Ignoring tank reserve. Do not plan to run a tank dry.
- Using generic figures over maker data. Check the spec sheet.
Tips for using a generator runtime calculator
- Keep diesel fresh with a stabilizer and regular use.
- Run the set under real load during tests.
- Plan fuel deliveries before storm season.
Diesel generator fuel consumption at part load
Because of the fixed no-load fuel, an oversized generator wastes fuel. A 50 kW set carrying 10 kW burns about 1.76 gal/h, while a 20 kW set carrying the same 10 kW burns 1.09 gal/h. Size the set close to the real load with the generator size calculator, keeping enough margin for motor starts. Ratings are often in kVA; the kW to kVA calculator converts them at a 0.8 power factor.
Generator fuel usage cost vs grid power
At 4 per gallon, the 20 kW diesel example costs about 4.36 an hour to make 10 kWh, roughly 0.44 per kWh before maintenance. Grid power usually costs far less, which is why generators suit backup and remote use. Compare the figures for your loads with the energy cost calculator.
Diesel generator fuel consumption chart (gal/h)
| Generator size | 25% load | 50% load | 75% load | 100% load |
|---|---|---|---|---|
| 10 kW | 0.38 | 0.55 | 0.71 | 0.87 |
| 20 kW | 0.77 | 1.09 | 1.42 | 1.74 |
| 50 kW | 1.92 | 2.74 | 3.55 | 4.36 |
| 100 kW | 3.85 | 5.47 | 7.10 | 8.72 |
| 200 kW | 7.70 | 10.94 | 14.19 | 17.44 |
| 500 kW | 19.24 | 27.36 | 35.48 | 43.61 |
Generator Fuel Consumption Calculator FAQ
How much fuel does a generator use per hour?
A 20 kW diesel at half load uses about 1.09 gal (4.1 L) per hour. Larger sets and higher loads use more.
How long will a generator run on a tank?
Divide the tank size by the fuel per hour. A 100 gallon tank feeding a 20 kW diesel at 50% load lasts about 91 hours.
Does propane use more than diesel?
Yes, by volume. Propane holds less energy per gallon and spark-ignition engines are less efficient, so a propane set burns roughly twice the gallons.
What is diesel generator fuel consumption per kWh?
About 0.25 to 0.35 L per kWh near full load, and more at light loads.