Generator Runtime Calculator
How long will a generator run on a tank? This generator runtime calculator estimates fuel burn per hour and total runtime from your genset size, tank, load, and fuel type.
Generator run time = usable fuel ÷ fuel burn per hour. Burn ≈ kW × load × a fuel factor (about 0.09 gal/kWh gasoline, 0.07 diesel, 0.14 propane). A 22 kW unit at 50% load on propane burns about 1.5 gal/hr, so 100 gallons lasts about 65 hours. Restoration Equipment Supply’s calculator below does it for any size.
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About This Calculator
A generator runtime calculator estimates how long a unit runs on a tank of fuel. Burn rate rises with generator size and load. Diesel is the most efficient per kW, propane the least; running at 50% load stretches runtime versus full load. Pair this with our generator sizing calculator to pick the right unit first.
Is this generator runtime calculator accurate?
It gives a solid planning estimate based on typical fuel-burn rates per kWh and published fuel energy content. For a job we always confirm the exact equipment with you by phone before delivery.
Can I rent or buy the equipment?
Both. Restoration Equipment Supply rents and sells the full drying and power fleet across NJ, NY, CT, PA, NC, SC and Florida, with same-day delivery for emergencies. Browse the full catalog →
What other calculators do you have?
Use our equipment sizing calculator, generator sizing calculator, and the tools in the Calculators menu to build a complete package.
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How Long Will a Generator Run on a Tank of Fuel?
Divide the usable fuel by the hourly burn rate. Burn rate depends on generator size, how hard it is working, and the fuel’s energy content.
The generator run time formula
Fuel burn (gal/hr) = generator kW × load % × fuel factor. Run time (hours) = usable gallons ÷ fuel burn. This calculator uses fuel factors of about 0.09 gallons per kWh for gasoline, 0.07 for diesel and 0.14 for propane — typical for portable and standby gensets at partial load.
Propane generator run time
For propane you can also work in BTUs: run time = gallons × 91,500 BTU/gal ÷ the generator’s BTU/hr consumption at your load. Consumption is on the spec sheet; if not, kW × load × 0.14 × 91,500 gets you close. Remember propane tanks are filled to about 80%, so a 500-gallon tank holds roughly 400 usable gallons and a 250-gallon tank about 200.
| Fuel | Approx. energy | Calculator fuel factor |
|---|---|---|
| Propane | ~91,500 BTU per gallon | 0.14 gal per kWh |
| Gasoline | ~120,000 BTU per gallon | 0.09 gal per kWh |
| Diesel | ~137,000 BTU per gallon | 0.07 gal per kWh |
Energy figures from the U.S. Energy Information Administration. Diesel packs the most energy per gallon, which is why it runs longest per tank.
| Usable propane | 50% load (~1.5 gal/hr) | 100% load (~3.1 gal/hr) |
|---|---|---|
| 100 gal | ~65 hours | ~32 hours |
| 200 gal (250-gal tank) | ~130 hours | ~65 hours |
| 400 gal (500-gal tank) | ~260 hours | ~130 hours |
Worked example: portable on gasoline vs. diesel
A 20 kW unit at 50% load with 12 gallons: gasoline burns 20 × 0.5 × 0.09 = 0.9 gal/hr → about 13 hours. On diesel, 20 × 0.5 × 0.07 = 0.7 gal/hr → about 17 hours. Need to size the generator first? Use the generator sizing calculator, then see generator rental — units are delivered fueled with refueling available on longer deployments.
How do you calculate generator run time?
Run time = usable fuel ÷ gallons burned per hour. Burn ≈ kW × load × fuel factor (0.09 gasoline, 0.07 diesel, 0.14 propane, gallons per kWh). Real burn varies by engine and load.
How long will a propane generator run on 100 gallons?
A 22 kW generator at half load burns about 1.5 gal/hr, so 100 gallons lasts roughly 65 hours. At full load it is closer to 32 hours. Smaller generators run longer.
How long will a 500-gallon propane tank run a generator?
A 500-gallon tank holds about 400 usable gallons at an 80% fill. At ~1.5 gal/hr (22 kW at half load) that is roughly 260 hours, or about 11 days of continuous running.
Does load affect generator run time?
Yes, directly. Fuel burn rises with load, so a generator at 50% runs about twice as long as at 100%. Shedding big loads (dryer, range, water heater) during an outage stretches fuel.
Updated October 2026 · Reviewed by the Restoration Equipment Supply dispatch team
How Our Generator Runtime Calculator Works
Drying plans and equipment counts follow the IICRC S500 Standard for Professional Water Damage Restoration. Restoration Equipment Supply is an IICRC Certified Firm.
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