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Propane Generator Runtime Calculator
Choose your standby generator and propane tank to see how many hours and days of backup power you have. Fuel rates come straight from Generac and Kohler spec sheets.
How it works
Generator makers publish how much propane each model burns at half load and at full load. The calculator draws a straight line between those points for your load (Kohler also publishes 25% and 75%, so its curve has four points). Then it divides the propane you can use by that hourly rate.
Kohler lists LP use in cubic feet of vapor per hour; we convert with the factor printed on its spec sheets, 36.39 cubic feet per gallon. For Generac loads below 50%, the line is extended past the published points, so treat those figures as rougher estimates.
Published LP fuel consumption
| Generator | 50% load | 100% load | Gallons/day at 50% |
|---|---|---|---|
| Generac Guardian 10kW | 0.97 gal/hr | 1.48 gal/hr | 23 |
| Generac Guardian 14kW | 1.81 | 3.07 | 43 |
| Generac Guardian 16kW | 2.03 | 2.94 | 49 |
| Generac Guardian 18kW | 1.70 | 3.02 | 41 |
| Generac Guardian 20kW | 2.37 | 3.56 | 57 |
| Generac Guardian 22kW | 2.53 | 3.90 | 61 |
| Generac Guardian 24kW | 2.53 | 3.90 | 61 |
| Generac Guardian 26kW | 2.06 | 3.63 | 49 |
| Kohler 14RESA | 1.65 | 2.23 | 40 |
| Kohler 20RESC | 2.25 | 3.74 | 54 |
Generac figures are from its current Guardian spec sheets; older model years can differ slightly, so check the fuel data decal on your unit. A bigger generator isn't always thirstier at half load (compare the 18kW and 26kW with their neighbors), because engines and tuning differ from model to model.
What "load" really means in an outage
Load is the share of the generator's rated output your house is drawing. Fridges, a furnace blower, lights and electronics might total 3 to 5 kW. A well pump, electric range, dryer or central air conditioner adds big spikes. Because those large loads cycle on and off, the average over a day is usually well below the peak. That's why 50% is a common planning figure. If you only back up essential circuits, 25–40% may be closer. If you run central air through a summer outage, plan nearer 75%.
You can stretch your propane by switching off non-essential loads, running the dryer and dishwasher only once in a while, and letting load-shedding modules hold off the air conditioner or water heater.
Can my tank keep up? Vaporization in cold weather
A propane tank doesn't just store fuel; it has to boil liquid into vapor fast enough to feed the engine. That boiling draws heat from the air around the tank, so in cold weather a small or nearly empty tank can't supply as much. The Propane Education & Research Council lists, for a half-full aboveground tank under intermittent draw at 0°F, about 181,000 Btu/hr for a 250 gallon tank, 300,000 for a 500 and 535,000 for a 1000. A Generac 22kW at full load needs about 357,000 Btu/hr (3.90 gal × 91,452). PERC adds that continuous draw cuts these limits sharply. That is why the calculator flags when your generator could outrun the tank, and why 100 lb cylinders only suit small generators. Your installer should size the tank and regulators for your coldest weather.
Safety
Standby generators must be installed outdoors with the clearances in the manufacturer's manual, connected through a transfer switch by a licensed electrician, and piped by a qualified gas installer. Never run a portable generator in a garage, basement or near open windows; carbon monoxide from generators kills people every year.
Frequently asked questions
How long will a 500 gallon propane tank run a 22kW Generac?
A 500 gallon tank filled to 80% holds 400 gallons. Using 300 of them (down to 20%), a Generac Guardian 22kW runs about 119 hours, roughly 5 days, at half load (2.53 gal/hr) and about 77 hours, 3.2 days, at full load (3.90 gal/hr).
How long will a 100 lb propane tank run a generator?
A full 100 lb cylinder holds about 23.6 gallons. That is about 24 hours for a 10kW Generac at half load, but only about 9 hours for a 22kW at half load. A single 100 lb cylinder also can't vaporize propane fast enough for a large standby generator in cold weather.
How much propane does a whole-house generator use per day?
Running nonstop at half load, a Generac 22kW uses about 61 gallons a day, a 14kW about 43, and a 10kW about 23. Real outages often average below half load, because big loads like a well pump or air conditioner cycle on and off.
What size propane tank do I need for a standby generator?
It depends on the generator, how long an outage you want to cover, and your other propane appliances. 500 gallon and larger tanks are common for 20kW-class generators. Besides capacity, the tank must be able to vaporize propane fast enough in your coldest weather; your installer sizes this.
Does a generator use propane when it exercises?
Yes, but not much. Generac's air-cooled units run a 5-minute exercise (weekly, or every two weeks or monthly on current models), much of it in a low-speed quiet mode. Over a year that adds up to a few gallons, small next to a single day of outage.
Sources
- Generac, Guardian 10/14/18 kW spec sheet, 16/20/22 kW spec sheet, 20/22/24 kW spec sheet (24kW: 2.53 and 3.90 gal/hr), 26 kW spec sheet: LP fuel consumption at 1/2 and full load, 5-minute exercise.
- Kohler, 14RESA spec sheet G4-245 and 20RESC spec sheet G4-246: LPG consumption in cfh at 25/50/75/100% load; 36.39 cu ft = 1 gal.
- Propane Education & Research Council, Propane Technical Pocket Guide, Table 5: maximum intermittent withdrawal rates by tank size and temperature.
- EIA, Energy units: 91,452 Btu per gallon of propane.
Last reviewed October 25, 2026. Estimates only; see the disclaimer.