Generator Runtime Calculator
Enter your generator's size, its fuel, how hard you'll load it and how much fuel you have, and get the gallons per hour, the hours per tank and the gallons a full day takes.
Generator runtime
E01Scenario A
Runtime on one tank
0.55 gal/h at 50% load; 13.2 gallons a day around the clock.
- Fuel use
- 0.55 gal/h
- Runtime per tank
- 10.9 h
- Gallons per 24 hours
- 13.2
Estimate. Formula and sources are under “How it works”. Change any input; the result updates as you type.
Formula
How it works
Runtime depends on the load, not the size printed on the generator. The calculator starts from the typical fuel use of a generator that size at half load, then scales it to your load:
- Gallons per hour = gallons per hour at 50% load for that size and fuel × (your load ÷ 0.5)^0.8
- Hours per tank = gallons on hand ÷ gallons per hour
- Gallons per 24 hours = gallons per hour × 24
The 0.8 exponent reflects that a generator gets a little more efficient as you load it: doubling the load doesn't quite double the fuel. At 25% load a 5,500 W gasoline generator uses an estimated 0.32 gal/h; at full load, 0.96 gal/h. The half-load figures come from manufacturers' spec sheets and are matched to the nearest size in our table.
Sources: fuel use at 50% load averaged from manufacturer spec sheets (Generac, Champion, Honda, Westinghouse). The 0.8 exponent is our approximation of the part-load fuel curve.
Worked example
A worked example
A 7,500 W generator on propane carrying 75% of its rating burns an estimated 1.66 gal/h. Twenty gallons of propane lasts about 12.0 hours at that load. Running it around the clock takes 39.8 gallons a day.
Questions
FAQ
How long will a generator run on 5 gallons of gas?
At half load, a 5,500 W gasoline generator uses about 0.55 gal/h, so 5 gallons lasts an estimated 9.1 hours. Load it lighter and it runs longer: at 25% load, the same 5 gallons lasts about 15.8 hours.
Propane vs gas runtime: which lasts longer?
Gasoline, per gallon. Propane holds less energy per gallon, so a 5,500 W generator burns about 0.90 gal/h of propane at half load against 0.55 gal/h of gasoline. Propane's advantage is storage: it doesn't go stale in the tank, and a larger tank is easy to keep full before storm season.
Does load affect fuel use?
Yes, almost in proportion. Going from 50% to 100% load raises fuel use by about 75% in this model, not 100%. The cheapest way to stretch a tank is to run only what you need at the same time.
Does the generator's size matter if the load is small?
Yes. A big generator at a light load still burns more than a small one at the same wattage, because the engine itself is bigger. Size for the load you'll actually run with the portable generator size calculator.
Why isn't my generator's number exactly this?
Engines, altitude, temperature and fuel quality all change consumption. Your manual lists run time at a stated load; if it gives a figure, compare it with this estimate and type your own tank size.
Related