How Long Will Propane Run a Standby Generator?
How long propane runs a standby generator depends on two numbers: how many gallons are in the tank and how many gallons an hour the generator burns at the load it is carrying. Generac publishes the burn rate for every Guardian size at half and full load.
Hours per 100 gallons
| Size | Gal/h half load | Gal/h full load | Hours per 100 gal, half | Hours per 100 gal, full | Gallons per day, full |
|---|---|---|---|---|---|
| 10 kW | 0.97 | 1.48 | 103.1 | 67.6 | 35.5 |
| 14 kW | 1.81 | 3.07 | 55.2 | 32.6 | 73.7 |
| 18 kW | 1.7 | 3.02 | 58.8 | 33.1 | 72.5 |
| 20 kW | 2.36 | 3.74 | 42.4 | 26.7 | 89.8 |
| 22 kW | 2.53 | 3.9 | 39.5 | 25.6 | 93.6 |
| 24 kW | 2.53 | 3.9 | 39.5 | 25.6 | 93.6 |
| 26 kW | 2.06 | 3.63 | 48.5 | 27.5 | 87.1 |
Figures from Generac's fuel table, Guardian Series. Run time scales directly: 400 gallons lasts four times as long as 100.
Half load is not half the fuel
At half load a 22 kW Guardian burns 2.53 gallons an hour against 3.90 at full load: 65% as much fuel for half the output. The same pattern holds across the line, from 56% for the 18 kW to 66% for the 10 kW. Cutting the load stretches a tank, but by about half again, not double.
A worked example
A 22 kW Guardian on propane, a tank holding 400 gallons, an outage where the house averages half load:
- 400 ÷ 2.53 = 158 hours, about six and a half days.
- If the load ran at full output the whole time: 400 ÷ 3.90 = 103 hours, about four days.
Real outages sit between the two, swinging between the full load of air conditioning or cooking and the light load of a quiet night. Generac's continuous-use guidance also calls for a shutdown of at least 30 minutes after every 24 hours of running, which adds a little to how long a tank lasts.
Bigger is not always thirstier
The 26 kW Guardian burns less propane than the 22 and 24 kW at both loads in Generac's table: 2.06 and 3.63 gallons an hour against 2.53 and 3.90. Generator size and fuel use do not rise in lockstep, so compare the table for the specific models you are quoted.
Natural gas
On a natural gas line there is no tank to run dry, but the burn rates still matter for sizing the gas meter and pipe. Generac's spec sheets note: "Fuel pipe must be sized for full load." The standby pages list cubic feet per hour for each size.
To see which size your list needs on propane, set the fuel in the calculator. How long propane will run a generator comes down to gallons in the tank divided by the hourly burn at your load.
Sources
- Generac Support, How much fuel does my home standby generator use (Guardian Series table, Feb 12, 2026)
- Generac Guardian 10/14/18 kW spec sheet
- Generac Support, Continuous use maintenance for home standby generators
Questions
How long will 100 gallons of propane run a 22 kW generator?
By Generac's fuel table, about 25.6 hours at full load (3.90 gallons an hour) and 39.5 hours at half load (2.53 gallons an hour).
Does half load double the run time?
No. A Guardian at half load burns roughly 56 to 66% as much propane as at full load, so the run time rises by about half, not double.
How much propane is in my tank?
Read the tank's gauge, which shows percent full, and multiply by the tank's water capacity in gallons.