How Long Will a Generator Run on Propane? Runtime Math (2026)

Most portable generators run about 7 to 10 hours on a full 20-pound propane tank at half load, and the biggest classes land at the bottom of that range: Westinghouse rates its 9,500-watt dual-fuel at up to 7 hours. The number is not a spec to memorize. It is the propane in the tank divided by the engine's burn rate, and both halves are findable in about a minute.

The short version:

• Plan 7 to 10 hours per full 20-pound tank at half load, and roughly half that at full load.
• A 6,750-watt propane generator burns about 2 to 3 pounds per hour at 50 percent load, the yardstick Lowe's publishes.
• A full 20-pound tank holds about 4.7 gallons, because propane weighs about 4.24 pounds per gallon.
• Tank size scales it: a 100-pound cylinder is five 20s' worth of fuel, and a 500-gallon home tank runs for weeks.

How long will a generator run on propane?

A mid-size portable runs about 7 to 10 hours on a 20-pound grill tank at half load, and a full-size 9,500-watt unit closer to 7. Applied to the tank sizes people actually own:

Tank	Propane when full	Half-load runtime	What it covers
20 lb cylinder	20 lb, about 4.7 gallons	7 to 10 hours	An overnight run on a mid-size unit
100 lb cylinder	100 lb, about 23.5 gallons	33 to 50 hours	Two days of half-load running
500 gal home tank	400 gallons at 80% fill	3 to 5 weeks	Whole-home standby territory

Those hours assume a 6,750-watt-class engine burning 2 to 3 pounds per hour. Bigger engines burn faster, so the Westinghouse WGen9500DFc 12500W Dual Fuel Generator is rated up to 7 hours on a 20-pound tank at half load; a small inverter burns less and stretches the same tank further. Load is the wild card: half load means half the unit's running watts, and double it and the hours roughly halve.

The two numbers behind every estimate

Every runtime reduces to hours = usable propane in pounds divided by burn rate in pounds per hour. Find the rate in the manual's runtime table, or use the published yardsticks: about 2 to 3 pounds per hour at half load for the 6,750-watt class, less for a small inverter, more for a whole-home unit. If the manual gives hours instead of a rate, divide, and a 20-pound tank lasting 7 hours works out to about 2.9 pounds per hour.

Then scale by tank. That 2.9-pound rate on a 100-pound cylinder gives about 34 hours. On a 500-gallon tank filled to 400 gallons, you are working with about 1,700 pounds of propane, close to four weeks of continuous half-load running, and heavier loads cut that fast.

Propane versus gasoline in the same machine

The trade shows up in the ratings: the mid-size Westinghouse WGen5300DFc 6600W Dual Fuel Generator runs 5,300 watts on gasoline and 4,800 on propane, so plan loads against the propane column when that is the fuel you will burn. What you get back is a fuel with no clock on it. Gasoline degrades in 30 to 90 days untreated and needs stabilizer and rotation (the full storage routine covers it); propane waits in the tank for years and does not varnish a carburetor. Refill at a dealer to get the full 20 pounds, or accept a 15-pound exchange fill for convenience.

The winter catch: vaporization

Propane is stored as a liquid and burned as a gas, and cold slows the boil between the two. A full tank has more liquid surface working for it, so fill ahead of a cold snap, keep the cylinder upright on a dry base out of the wind, and expect the biggest loads, like a well pump or a large compressor, to feel a small tank's winter limits first. Deep-winter duty is a good reason to step up to a 100-pound tank like the Flame King YSN100b 100-LB Steel Propane Tank with… (about $269), or to keep two 20s and rotate them. One catch: 100-pound cylinders use a POL valve, while most generator hoses fit the QCC1 fitting on a 20-pound tank, so add a brass POL-to-QCC1 adapter like the Flame King POL to QCC1 Propane Tank Adapter (about $12), which threads onto the tank's POL valve and gives the hose the QCC1 fitting it needs.

Running the tank safely

Swap tanks with the generator off and cooled, and hand-tighten the hose fitting; leak-check with soapy water, never a flame. Spare cylinders stay outdoors or in a well-ventilated outbuilding, away from exhaust. The other rule rides along with every fuel: run the generator outside only, at least 20 feet from doors, windows, and vents, per CPSC guidance, with a battery CO alarm indoors, as the generator CO guide details. The gasoline runtime guide handles the same math for the other fuel.

Quick recommendations

• Fridge, freezer, and a furnace blower, around $649: the Westinghouse WGen5300DFc 6600W Dual Fuel Generator is the mid-size dual-fuel pick, and its propane side covers a real essentials load.
• Whole house minus central AC, around $1,049: the Westinghouse WGen9500DFc 12500W Dual Fuel Generator gives up to 12 hours on gasoline, or about 7 on a 20-pound propane tank at half load.
• Fuel that is ready when the grid is not: a refillable Flame King YSN230 20-LB Steel Propane Tank with… (about $70) has a built-in gauge, so you know where the tank stands before the forecast turns.

Fill the tank before the weather turns, and propane keeps its side of the bargain: it waits, season after season, until you need it.