How much propane does a generator use?
Published October 3, 2026.
Built by Jeremy Panasuk.
This is the home calculator's runtime, for gasoline and for propane. It is not a second formula, and it is not the storm runbook's button.
sizeGenerator() in assets/app.js sets gasoline gallons per hour = recommended watts ÷ 7,000, then propane gallons per hour = that gasoline rate × 1.37. A 3,500 W recommendation and a 4 gallon tank print 8.0 hours on gasoline and 5.8 hours on propane. At 8 hours a day the propane line is 23.2 lb/day, using the script's 4.24 lb/gal factor.Two scripts, two propane factors
The home calculator and the storm runbook both start from one gallon of gasoline per 7,000 watts per hour. They do not apply that rule to the same watt number, and they do not use the same propane multiplier.
| Home calculator | Storm runbook | |
|---|---|---|
| Watts in the 7,000 | Recommended watts, after surge and 20% headroom | Running watts you type |
| Propane volume | Gasoline gallons × 1.37 | Gasoline gallons ÷ 0.75 (1.33×) |
| Pounds | Propane gal/h × hours/day × 4.24 | Not printed |
EIA's sample heat contents are 120,214 Btu per gallon of motor gasoline and 91,452 Btu per gallon of propane. 120,214 / 91,452 = 1.314, so the same heat needs about 31% more propane volume than gasoline. Neither 1.37 nor 1.33 is that ratio. Both are script factors. A manufacturer fuel table, such as the gallons per hour on the 20 kW Guardian sheet, is a third number and is not produced by either script.
Worked example: the 3,500 W food-truck recommendation
The watt input is the recommendation on best generator for a food truck, not a new measurement. Honda's Energy Star fridge at 192 W running / 1,200 W start, a 4-cup coffee maker at 600 / 600, and a 650 W microwave at 1,000 / 1,000 produce a 3,500 W recommendation after surge and 20% headroom. Tank 4 gallons. Hours per day 8, which is the calculator's default, not a claim that the truck draws 3,500 W for eight hours.
- Gasoline gal/h = 3,500 / 7,000 = 0.5. Hours on 4 gallons = 4 / 0.5 = 8.0. Gallons per day = 0.5 × 8 = 4.0.
- Propane gal/h = 0.5 × 1.37 = 0.685. Hours on the same 4 gallons = 4 / 0.685 = 5.8, because the script prints one decimal.
- Pounds per day = 0.685 × 8 × 4.24 = 23.2.
The comment above sizeGenerator() says a 20 lb tank holds 4.7 gallons. That 4.7 is the comment's figure (20 / 4.24 is about 4.72). It is not a tank this page weighed. Entering 4.7 gallons at the same 3,500 W prints 9.4 hours of gasoline and 6.9 hours of propane. Pounds per day stay 23.2, because that line uses hours per day, not tank size.
If the 7,000 were applied to the 1,792 W running total instead of the 3,500 W recommendation, gasoline would be 1,792 / 7,000 gallons per hour and the tank would last longer. That is what the storm runbook does. Putting 3,500 into the runbook treats the recommendation as if it were running watts.
Runtime calculator
Type a recommended wattage, a tank in gallons, and hours per day. The button repeats the home calculator's fuel lines only. It does not add surge, and it does not apply the runbook's ÷ 0.75.
A common mistake
Reading "~25% less runtime" off a comparison table and using it in place of the button. EIA's heat ratio is about 31% more propane volume (divide by 1.314, not by 1.37). The home button is 1.37. The runbook is ÷ 0.75. Three different statements. The other mistake is dividing a 20 lb tank's pounds by 4.24 and calling the result an EIA density. EIA's page cited here publishes Btu per gallon. The 4.24 figure is the factor in the pounds-per-day line of sizeGenerator().
Limits, and when to call a pro
These hours assume a steady load at 50% of the recommended watts. A fridge that cycles off burns less. A generator manual that prints its own hours at 50% load, such as a tested tank time on that model, is the number for that machine. Do not "correct" the manual with 1.37, and do not "correct" this button with the manual.
A propane tank piped to a standby generator, a regulator, and a transfer switch are a gas-fitting and electrical installation. That takes a licensed electrician and a permit where local code requires one. Portable placement is unchanged: outdoors, at least 20 feet from the house, on how to run a generator safely. Propane does not make an indoor generator safe.
Replay the watt step on the generator size calculator before you type a recommendation into the box above. The kW to kWh calculator turns watts and hours into energy. It does not turn these gallons into kilowatt-hours.
Compiled with AI assistance from the sources below. Spot an error? Email [email protected].
Sources
- EIA, British thermal units — 120,214 Btu/gal motor gasoline and 91,452 Btu/gal propane
- Honda, What wattage generator do I need? — the appliance rows behind the 3,500 W recommendation
- GeneratorSizer calculator script —
sizeGenerator(), the 7,000, 1.37, and 4.24 factors