How much propane does a whole-house generator use?

The short answer: a typical 20 to 24 kW home standby generator burns about 2.5 gallons of propane an hour at half load and about 3.5 to 4 gallons an hour at full load. Smaller units burn less. A 10 kW unit sips under a gallon an hour at half load.

That number matters for one reason. On propane, the tank is the ceiling. The generator runs until the tank runs out, so the burn rate and the tank size together decide how long you stay powered in a long outage.

Burn rates from the spec sheets

Manufacturers publish fuel consumption at half load and full load on every spec sheet. These are the propane figures in gallons per hour, straight from the current Generac Guardian and Kohler sheets:

Generator	½ load	Full load
Generac 10 kW	0.97	1.48
Generac 14 kW	1.81	3.07
Generac 18 kW	1.70	3.02
Generac 22 kW	2.53	3.90
Generac 24 kW	2.53	3.90
Kohler 20RCAL	about 2.2	about 3.7

Kohler publishes its figures in cubic feet per hour of vapor (82 ft³/hr at 50% load, 136 ft³/hr at 100%). We converted those to gallons using Kohler’s own rating of 2,500 Btu per cubic foot and the EIA figure of 91,452 Btu per gallon, so treat the Kohler row as a close approximation rather than a published number.

Two things stand out in the table:

• The 22 kW and 24 kW burn the same. They share Generac’s 999 cc engine, and the spec sheet gives identical propane figures for both.
• The 14 kW burns slightly more than the 18 kW at half load. They share an 816 cc engine, so the 18 kW at half load is carrying more power for about the same fuel. Burn rate tracks the engine and how hard it’s working, not the number on the label.

Why half load is the number to plan with

Full load is the worst case: every circuit the generator covers, drawing at once. Real outages rarely look like that. The air conditioner cycles, the water heater finishes, the well pump runs for a minute and stops. Most of the time a properly sized generator is working well under its rating.

That’s why the half-load figure is the more realistic planning number, and the full-load figure is your conservative bound. If you plan with both, you know the range your tank has to cover.

Sizing to your real loads helps twice here. It picks the right kW, and it tells you roughly what load the generator will actually run at.

How long a tank lasts

A propane tank is named by its water capacity, but it’s only filled to about 80%. The empty space lets the liquid expand as it warms. So a 500 gallon tank holds about 400 gallons of propane when it’s full.

Divide usable gallons by the burn rate and you get hours of run time. For a Generac 22 kW, using the spec-sheet rates above (2.53 gallons an hour at half load, 3.90 at full load):

Tank	Usable	½ load	Full load
250 gal	200 gal	79 hr (3.3 days)	51 hr (2.1 days)
500 gal	400 gal	158 hr (6.6 days)	103 hr (4.3 days)
1,000 gal	800 gal	316 hr (13.2 days)	205 hr (8.5 days)

For comparison, the same 400 gallons runs a 10 kW unit at half load for about 412 hours, or 17 days.

These numbers assume the tank starts full and the generator runs nonstop. In practice the tank is rarely full when the outage starts, and the generator may shut off when the grid comes back for a few hours. Use your tank gauge reading, not the tank size, when you do the math during an outage.

Work out your own number

You need three figures:

1. Your generator’s propane burn rate at half and full load, from its spec sheet. Every model page in our standby catalog links the manufacturer’s product page, where the spec sheet is posted.
2. Usable gallons in the tank. Read the gauge (it shows percent of tank capacity), multiply by the tank’s size. A 500 gallon tank reading 60% holds about 300 gallons.
3. Hours you want to cover. Think about the longest outage your area has had, and add margin.

Then: usable gallons ÷ burn rate = hours of run time. Running cost works the same way: burn rate × your price per gallon = cost per hour.

If the spec sheet only lists cubic feet per hour, multiply by 2,500 to get Btu per hour, then divide by 91,452 to get gallons per hour. That’s how we converted the Kohler figures above.

Exercise and standby use are small

A standby generator starts itself on a schedule to keep the engine and battery ready. That exercise burns fuel, but not much. Kohler’s 20RCA(L) sheet lists about 35 cubic feet per hour during exercise, roughly 1 gallon an hour, and its unloaded exercise runs 20 minutes. Once a week that adds up to about a third of a gallon, or around 17 gallons a year. Generac’s Quiet-Test exercise runs 5 minutes at low speed. See the maintenance guide for what else the exercise cycle checks.

What this means for choosing a tank

If you’re installing propane for a generator, pick the tank from the outage you want to ride out, not from the generator alone. A 250 gallon tank carries a 22 kW unit for about three days at half load, which covers most storm outages but not a long regional one. A 500 gallon tank roughly doubles that. Many homes also run a furnace, water heater, or range on the same tank, which shortens the numbers above.

If you already have a natural gas line, the tank question goes away: the generator runs as long as the line holds pressure. The tradeoffs between the two fuels are in natural gas vs. propane.

Once you know the run time you need, compare standby models by their propane burn rate and check the installed cost, which should include the tank, the regulator, and the gas line to the pad.