Generator fuel and runtime calculator
Generator spec sheets give one run time at one load. Enter it, plus your load and the fuel you have; the calculator works out the generator’s efficiency from the spec and scales it to your situation using each fuel’s energy content.
Enter your measurements
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
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
Quick answers for common sizes
| Example | Answer |
|---|---|
| How long a generator runs on 5 gallons of gas | About 14.9 hours (3500 W generator at half load) |
| How long a generator runs on a 20 lb propane tank | About 10.5 hours (3500 W dual-fuel at half load) |
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 |
Worked examples: how long a generator runs on 5 gallons of gas and how long a generator runs on a 20 lb propane tank.
How the estimate works
A generator turns the heat in its fuel into electricity at some efficiency. The spec sheet tells you that efficiency indirectly: it ran so many hours at a stated load on a stated amount of fuel. Using the EIA’s heat content for each fuel, the calculator works out that efficiency, then uses it to find the fuel your load needs.
efficiency = (rated W × spec load × spec hours ÷ 1,000 × 3,412 Btu/kWh) ÷ (spec fuel in gallons × Btu per gallon) fuel per hour = your load W ÷ 1,000 × 3,412 ÷ efficiency ÷ Btu per gallon runtime = your fuel ÷ fuel per hour
| Fuel | Heat content (EIA) | Notes |
|---|---|---|
| Gasoline | 120,214 Btu per gallon | Goes stale in storage; use stabilizer |
| Diesel | 137,381 Btu per gallon | Most energy per gallon |
| Propane | 91,452 Btu per gallon | About 4.23 lb per gallon at 60 °F (NIST), so a 20 lb tank holds about 4.7 gallons |
Spec-sheet run times
| Generator (Champion) | Fuel | Spec run time | Implied fuel use |
|---|---|---|---|
| 3500 W dual fuel | Gasoline, 4.7 gal | 14 h at 50% load | 0.34 gal/h at 1,750 W |
| 3500 W dual fuel | Propane, 20 lb | 10.5 h at 50% load | 1.9 lb/h at 1,575 W |
| 6500 W dual fuel | Gasoline, 7.7 gal | 11 h at 50% load | 0.7 gal/h at 3,250 W |
| 6500 W dual fuel | Propane, 20 lb | 6.5 h at 50% load | 3.08 lb/h at 2,925 W |
| 2000 W inverter | Gasoline, 1.05 gal | 11.5 h at 25% load | 0.091 gal/h at 425 W |
| 2000 W inverter | Propane, 20 lb | 34 h at 25% load | 0.59 lb/h at 383 W |
Why light loads burn more than you’d think
An engine burns fuel just to keep turning, so fuel use doesn’t fall in proportion to the load. This calculator assumes the spec-sheet efficiency holds at your load, which is close near the spec load and optimistic well below it. Inverter generators, which throttle the engine down at light loads, hold their efficiency better than open-frame units running at a fixed speed.
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) - Propane tank gaugeBudget price
Shows roughly how much is left so you are not guessing halfway through a cold night or a long outage.
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)
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 generator run on 5 gallons of gas?
It depends on the generator and the load. Champion’s 3500 W dual-fuel model is rated 14 hours at half load on 4.7 gallons, so 5 gallons is about 14.9 hours at 1,750 W. See how long a generator runs on 5 gallons of gas.
How much propane does a generator use per hour?
Champion’s 3500 W dual-fuel model is rated 10.5 hours at half load on a 20 lb tank — about 1.9 lb (0.45 gallons) an hour. Its 6500 W model uses about 3.1 lb an hour at half load. See how long a generator runs on a 20 lb propane tank.
Does a generator run longer on gas or propane?
Gasoline usually goes further per gallon — it holds about 120,214 Btu against propane’s 91,452 (EIA). Propane’s advantage is storage: it doesn’t go stale, and a few 20 lb tanks are easy to keep on hand.
How much gas does a generator use in 24 hours?
Multiply the fuel per hour by 24. A 3500 W generator at half load using about 0.34 gallons an hour burns about 8 gallons a day.
Related guides
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.