How long will a generator run on 5 gallons of gas?
A 3500 W generator rated 14 hours at half load on its 4.7-gallon tank (Champion’s dual-fuel model) runs about 14.9 hours on 5 gallons at 1,750 W — burning about 0.34 gal an hour.
How to work it out
- Spec: 3,500 W × 50% × 14 h = 24.5 kWh from 4.7 gal × 120,214 Btu/gal → 15% efficient
- Your load: 1,750 W × 3,412 Btu/kWh ÷ 15% = 40,358 Btu an hour = 0.336 gal of gasoline an hour
- 5 gal ÷ 0.336 gal/h = 14.9 hours
Five gallons is the size of a standard gas can, so this is the question people ask during an outage. The honest answer starts from your own generator’s spec sheet; this page uses Champion’s 3500 W dual-fuel model as the example.
Adjust for your project
More options: Spec load · Spec hours 2
- In hours and minutes
- 14 h 54 min
- Fuel per hour
- 0.34 gal
- Fuel per 24 hours
- 8.1 gal
- Generator efficiency (fuel to electricity)
- 15%
- Fuel energy burned per hour
- 40,358 Btu
Runtime at other loads
| Your load | Runtime at your load | Fuel per hour |
|---|---|---|
| 500 | 52.1 hours | 0.1 gal |
| 1000 | 26.1 hours | 0.19 gal |
| 1750 | 14.9 hours | 0.34 gal |
| 2500 | 10.4 hours | 0.48 gal |
| 3500 | 7.4 hours | 0.67 gal |
What changes the answer
- Size of the generator. Champion’s 6500 W model is rated 11 hours at half load (3,250 W) on 7.7 gallons — about 0.7 gallons an hour, so 5 gallons lasts about 7 hours at that load.
- Load. At a quarter load, the same 3500 W generator would run longer — but not twice as long, because the engine burns fuel just to turn. With one spec point the calculator shows a range; enter a second spec-sheet run time to narrow it.
- Inverter vs open frame. Champion’s 2000 W inverter is rated 11.5 hours at 25% load on 1.05 gallons — under a tenth of a gallon an hour — because it throttles down.
- Old gas. Gasoline goes stale in storage; stale fuel makes engines run poorly. Rotate stored cans or use a stabilizer.
Burning about 8 gallons a day at half load adds up in a long outage. Compare propane on how long a generator runs on a 20 lb propane tank, or a quiet battery on how many kWh to power a house for a day. To check the generator is big enough in the first place, see what size generator to run a house.
Different size? Use the full Generator Fuel & Runtime Calculator.
What you’ll need
- Dual-fuel portable generatorPremium price
Runs on gasoline or propane. Propane stores for years without going stale; expect somewhat fewer watts on propane.
See options on Amazon (opens in a new tab) - Battery-operated carbon monoxide alarmBudget price
CPSC advises battery or battery-backup CO alarms whenever a generator is in use. Look for UL 2034 certification and test monthly.
See options on Amazon (opens in a new tab) - Outdoor generator extension cordMid-range price
Lets the generator sit well away from the house. CPSC says to use outdoor-rated cords whose rating exceeds the total load, with all three prongs.
See options on Amazon (opens in a new tab)
As an Amazon Associate we earn from qualifying purchases. Links to products are affiliate links — they cost you nothing extra and help keep these calculators free.
Frequently asked questions
How long will a 3500 W generator run on 5 gallons at a quarter load?
Somewhere between 14 h 54 min to 29 h 47 min: the lower figure assumes it burns as much as at half load, the higher assumes constant efficiency. An engine burns fuel just to keep turning, so the truth lies in between.
How long will a 6500 W generator run on 5 gallons of gas at half load?
About 7.1 hours, using about 0.7 gal an hour.
How much gas does a 3500 W generator use in 24 hours at half load?
About 8.1 gal.
Other common sizes
- How long a generator runs on a 20 lb propane tank — About 10.5 hours (3500 W dual-fuel at half load)
Sources
- U.S. Energy Information Administration — British thermal units (Btu) (opens in a new tab) — Heat content: gasoline 120,214 Btu/gal, diesel 137,381 Btu/gal, propane 91,452 Btu/gal; 1 kWh = 3,412 Btu.
- NIST Chemistry WebBook — Thermophysical properties of propane (saturated liquid) (opens in a new tab) — Liquid propane is about 31.6 lb/ft³ at 60 °F, which is 4.23 lb per US gallon (4.09 at 80 °F).
- Champion Power Equipment — 3500-Watt Dual Fuel Generator (model 201213) specifications (opens in a new tab) — 3,500 running / 4,000 starting watts on gasoline; 14 h at 50% load on 4.7 gal; 10.5 h at 50% load on a 20 lb propane tank; 120 V 30 A TT-30R RV outlet.
- Champion Power Equipment — 6500-Watt Dual Fuel Generator (model 201077) specifications (opens in a new tab) — 6,500 running / 8,125 starting watts on gasoline (5,850 / 7,325 on propane); 11 h at 50% load on 7.7 gal; 6.5 h at 50% load on a 20 lb propane tank.
- Champion Power Equipment — 2000-Watt Dual Fuel Inverter (model 201230) specifications (opens in a new tab) — 1,700 running / 2,000 starting watts on gasoline (1,530 / 2,000 on propane); 11.5 h at 25% load on 1.05 gal; 34 h at 25% load on a 20 lb propane tank.
- U.S. Consumer Product Safety Commission — Portable Generator Hazards (Safety Alert 5123, 2021) (opens in a new tab) — Never run a generator indoors or in a garage, even with doors open; place it outside far from doors, windows and vents; CO alarms; never “backfeed” by plugging into a wall outlet.