Backup power
Generator fuel consumption calculator
A generator burns fuel in proportion to the power it delivers, but small engines are least efficient at light load. Roughly: a 7.5 kW portable at half load burns about 0.75 gallons of gasoline an hour, and a 22 kW standby at half load burns about 2.5 gallons of propane an hour.
0.75 gal/hgasoline at 50% load (3.8 kW)
- Per 24 h
- 17.9 gal
- For 24 h
- 17.9 gal ≈ $57.37
- One tank (6.0 gal)
- 8.0 h
- Fuel-to-electricity efficiency
- 14%
Estimate within about ±20%. Your generator's spec sheet lists fuel use at ½ and full load; when you have it, trust it over this.
How the estimate works
electrical output (BTU/h) = rated kW × load × 3,412 fuel input (BTU/h) = output ÷ efficiency(load, size) fuel per hour = fuel input ÷ BTU per gallon (or per ft³)
Fuel energy content comes from the U.S. Energy Information Administration: gasoline 120,214 BTU/gal, propane 91,452 BTU/gal, diesel 137,381 BTU/gal and natural gas about 1,036 BTU/ft³. The hard part is efficiency. A small air-cooled engine turns only 15–22% of the fuel's energy into electricity at full load, and much less at light load, because the engine still has to spin at speed. The calculator uses a full-load efficiency by size class (17% under 4 kW, 19% up to 13 kW, 22% for standby units) and a part-load curve that drops to about 60% of that at quarter load. That curve was checked against the fuel tables manufacturers publish at ¼, ½ and full load, and lands within roughly ±20% of them.
Fuel use by generator and load
| Generator | Fuel | 25% load | 50% load | 100% load |
|---|---|---|---|---|
| 2,200 W inverter | Gasoline | 0.15 gal | 0.24 gal | 0.37 gal |
| 7,500 W portable | Gasoline | 0.45 gal | 0.75 gal | 1.12 gal |
| 9,500 W portable on LP | Propane (LP) | 0.75 gal | 1.24 gal | 1.87 gal |
| 14 kW standby | Natural gas | 85 ft³ | 140 ft³ | 210 ft³ |
| 22 kW standby | Propane (LP) | 1.50 gal | 2.49 gal | 3.73 gal |
| 22 kW standby | Natural gas | 133 ft³ | 220 ft³ | 329 ft³ |
How long propane lasts
A 22 kW standby generator at half load burns about 2.49 gallons of propane an hour, 60 gallons a day. That puts tank sizing in perspective:
| Propane supply | Usable gal | Runtime at 50% load |
|---|---|---|
| 20 lb cylinder (grill tank) | 4.7 | 1.9 h (0.1 days) |
| 30 lb cylinder | 7.1 | 2.9 h (0.1 days) |
| 100 lb cylinder | 23.6 | 9.5 h (0.4 days) |
| 250 gal tank (filled to 80%) | 200.0 | 80.4 h (3.4 days) |
| 500 gal tank (filled to 80%) | 400.0 | 160.8 h (6.7 days) |
| 1,000 gal tank (filled to 80%) | 800.0 | 321.6 h (13.4 days) |
Load management stretches every number: a generator idling at 25% load does not burn a quarter of the full-load fuel, it burns closer to 40%. Running it only a few hours at a time to cool the fridge and pump the sump is far more fuel-efficient than running it all night.
Safety. Store gasoline only in approved containers, away from the house, and refuel a portable generator only after it has cooled. Gasoline goes stale in 3–6 months without stabilizer. Propane cylinders stay outdoors, upright.
Common questions
How much propane does a 22 kW generator use per day?
About 60 gallons a day at half load and close to 90 at full load. A 500-gallon tank filled to 80% (400 gallons) lasts about 6½ days at half load.
How long will a generator run on a 20 lb propane tank?
A 20 lb tank holds about 4.7 gallons. A 7,500 W dual-fuel generator at half load burns roughly 0.9–1 gal/h on propane, so 4½–5 hours.
Is propane or gasoline cheaper for a generator?
Propane carries about 24% less energy per gallon, so you burn more gallons. It wins on storage: it does not go stale and tanks can be large. Gasoline is cheaper per kWh produced at typical prices but has to be rotated.
Guides that use this calculator
- How Much Propane Does a Standby Generator Use?
- Generator Carbon Monoxide Safety: How Far From the House?