Last updated
How long common generators run on a tank
Gasoline, with the generic fuel curve below. Each cell shows the runtime at that share of the generator’s running watts.
| Generator and tank | 25% load | 50% load | 100% load |
|---|---|---|---|
| 2,200 W inverter, 1.1 gal | 6 h 8 min | 4 h 1 min | 2 h 58 min |
| 3,500 W portable, 4 gal | 14 h | 9 h 12 min | 6 h 48 min |
| 7,500 W portable, 6.6 gal | 10 h 47 min | 7 h 5 min | 5 h 14 min |
Runtime does not shrink in proportion to the load. A generator at 25% load burns more than a quarter of the fuel it burns at 100%, which is why oversized generators waste fuel on small loads.
How the runtime is calculated
- Load share = your load ÷ the generator’s running watts.
- Efficiency = 6% + 11% × load share, so about 9% at a quarter load, 11.5% at half and 17% at full load. It is a generic curve for small portable generators.
- Fuel per hour = load in kWh ÷ efficiency ÷ energy per gallon. A gallon holds about 35 kWh for gasoline, 40 for diesel and 26.7 for propane. A 20 lb propane tank holds about 4.7 gallons.
- Runtime = tank size ÷ fuel per hour.
- With your spec sheet: if you enter the rated runtime at 50% load, the fuel rate at 50% is tank ÷ that runtime, and other loads are scaled from it using the same curve.
It does not model altitude, temperature, engine age or the exact fuel curve of a particular model.
Ways to stretch your fuel
- Match the generator to the load. A big generator running a small load wastes fuel. An inverter generator that adjusts engine speed uses much less at light loads.
- Run big loads together, then shut down. Pump, wash and charge in one session instead of idling for hours.
- Add a battery. Let a battery cover small overnight loads and run the generator only to recharge it.
- Keep fuel fresh. Untreated gasoline can go stale in a few months. A fuel stabilizer helps, and propane keeps indefinitely.
Using it anywhere
- Tank size works in gallons, liters, or pounds of propane, and results are shown in both gallons and liters.
- Watts are the same on any voltage. Use the generator’s running rating, not its starting or peak rating.
Common mistakes
- Using the peak rating. Use running watts.
- Assuming linear fuel use. Light loads burn more fuel per kWh than moderate loads.
- Forgetting diesel at low load. Diesel engines run poorly at very light loads, which can cause buildup.
- Overfilling a hot generator. Let it cool before refueling.
Common questions
How long will a generator run on a tank of gas?
A 3,500 W generator with a 4-gallon tank running at half load, 1,750 W, lasts about 9 h 12 min by this estimate, and about 14 h at a quarter load. Check your spec sheet for the rated runtime at 50% load and enter it in the calculator for a closer figure.
How long will a 5-gallon can of gas run a generator?
At 1,750 W on a 3,500 W generator, a 5-gallon can lasts about 11 h 30 min. At a quarter load of 875 W it lasts about 17 h 30 min. Fuel use is roughly 0.43 gallons an hour at half load.
How much fuel does a generator use in 24 hours?
A 3,500 W generator at half load burns about 10.4 gallons of gasoline a day. At a quarter load it burns about 6.9 gallons. The calculator shows the fuel for 24 hours at your load, in gallons and liters.
How long will a 20 lb propane tank run a generator?
A 20 lb tank holds about 4.7 gallons of propane, with about 26.7 kWh per gallon. A 3,500 W generator at 1,750 W would use it in roughly 8 h 15 min. Propane gives fewer kWh per gallon than gasoline, so the same tank lasts less time.
What size generator should I buy?
Add up the running watts of everything you want to run, add about 20%, and check that the starting watts cover your biggest motor. A generator running at 40–75% of its rating is usually the most efficient. The power outage calculator and the inverter size calculator help with the sizing.
Is it bad to run a generator at a very low load?
It wastes fuel, because efficiency falls at light loads. For diesel engines it can also cause carbon buildup, known as wet stacking. If your load is small, a smaller or inverter generator, or a battery that the generator recharges, is usually better.
Related calculators
To size a generator or battery for a blackout, use the power outage calculator. Choose an inverter with the inverter size calculator, and compare with the battery runtime calculator.