How long will a generator run on a 20 lb propane tank? (Plus 30, 40 and 100 lb)

A 20 lb tank is the same size as a standard grill tank. How long it lasts depends on two things: how big the generator is and how much you plug in. The good news is that makers publish their own run times, so you don't have to guess.

How much propane is in a 20 lb tank?

Liquid propane weighs about 4.2 pounds per gallon at 60 °F. So the "lb" on a tank tells you roughly how many gallons it holds when full.

Tank	Propane weight	About how many gallons
20 lb	20 lb	4.8 gal
30 lb	30 lb	7.1 gal
40 lb	40 lb	9.5 gal
100 lb	100 lb	23.8 gal

Gallons = pounds ÷ 4.2. The weight is the propane inside, not the empty tank.

A 100-gallon tank is a different thing: a big fixed tank, not a 100 lb cylinder. Ask your propane supplier how much a full one holds.

Makers' run times on a 20 lb tank

Here are figures makers publish on their own spec pages. The watts shown are each model's running watts. Each one is linked in the sources below.

Generator (running watts)	Maker's 20 lb run time	30 lb	40 lb	100 lb
Small inverter (1,700 W on gas)	34 h at 25% load	~51 h	~68 h	~170 h
Mid-size inverter (3,150 W on propane)	21 h at 25% load	~32 h	~42 h	~105 h
Large inverter (7,000 W on gas)	12 h at 25% load	~18 h	~24 h	~60 h
Open frame (3,150 W on propane)	10.5 h at 50% load	~16 h	~21 h	~53 h
Open frame (8,500 W on propane)	7 h at 50% load	~10.5 h	~14 h	~35 h
Open frame (6,750 W on propane)	5.5 h at 50% load	~8 h	~11 h	~28 h

Only the 20 lb column is published. The other columns are our estimates:

Hours on a bigger tank ≈ maker's 20 lb hours × (tank lb ÷ 20)

Watch the load %. A figure "at 25% load" isn't comparable with one "at 50% load." Don't pick a generator on the bigger number alone.

Before you use a 100 lb tank, check your manual. Champion's FAQ, for example, recommends 20 to 40 lb tanks for its dual-fuel models.

Worked example

One maker's 9,500 W dual-fuel generator lists 7 hours on a 20 lb tank at 50% load. On propane it's rated 8,500 running watts, so 50% is about 4,250 W. That's enough for a fridge, a sump pump, a furnace fan and lights.

• 20 lb tank: 7 hours
• 40 lb tank: 7 × (40 ÷ 20) = 14 hours
• 100 lb tank: 7 × (100 ÷ 20) = 35 hours

Say you only run it 12 hours a day during an outage. A 100 lb tank would last about three days. Three 20 lb tanks would give you about 21 hours.

Use the calculator above. It's pre-filled with this generator's published numbers. Change the tank size or load to see your own estimate.

Why load % changes everything

The more you plug in, the faster the tank empties. But it isn't a straight line.

The same 9,500 W generator lists, on gasoline, 12 hours at 25% load and 8 hours at 50% load. Doubling the load cuts run time by a third, not by half. That's because the engine burns some fuel just to keep itself spinning.

So:

• Light loads stretch a tank. Unplug what you don't need.
• Heavy loads drain it fast. A big generator working hard can empty a 20 lb tank in 5 or 6 hours, as the last row of the table shows.
• Inverter generators gain the most at light loads. Their engines slow down when demand drops. See inverter vs conventional generators.

Why propane gives less power (the derate)

Dual-fuel generators make less power on propane. Makers show this as two running-watt ratings:

Generator	Running W on gas	Running W on propane	Drop
Open frame A	3,500	3,150	10%
Open frame B	7,500	6,750	10%
Open frame C	9,500	8,500	about 11%

Plan for about 10% less power on propane. If you're sizing for a well pump or a big fridge, size using the propane number. Our generator size calculator can help.

Propane also holds less energy per gallon. The U.S. Department of Energy lists propane at 84,250 Btu per gallon, about 0.74 of a gallon of gasoline. You can see it in the makers' figures. The 3,500 W model above gets about 3.0 hours per gallon of gas at 50% load, but about 2.2 hours per gallon of propane.

Why use propane, then? It doesn't go stale in storage the way gasoline can, and full tanks are easy to swap.

Safe tank habits

• Keep the tank outdoors, next to the generator. Never store or use a propane tank indoors or in a garage.
• Close the tank valve when you shut the generator down.
• Check the hose and regulator that came with the generator before each use.
• Swap tanks with the engine off.

A propane generator's exhaust still contains deadly carbon monoxide. Run the generator outdoors only, at least 20 feet from your home, with the exhaust pointed away. Read our generator carbon monoxide safety guide.