How long will a generator run on a 20 lb propane tank?
A full 20 lb tank holds about 4.7 gallons. Hours = 4.7 divided by the generator's propane use in gallons per hour. A small 3,500 W unit at half load burns about 0.45 gal/h and runs roughly 10.5 hours. A 7.5 kW unit at half load runs about 5.4.
Hours by burn rate
Find the gallons per hour in your generator's manual or spec sheet (look for the propane column at 50% and at 100% load), then read the hours here.
| Propane use | Hours on a full 20 lb tank |
|---|---|
| 0.25 gal/h | 18.9 h |
| 0.50 gal/h | 9.4 h |
| 0.75 gal/h | 6.3 h |
| 1.00 gal/h | 4.7 h |
| 1.50 gal/h | 3.1 h |
| 2.00 gal/h | 2.4 h |
The arithmetic
20 lb ÷ 4.24 lb per gal = 4.72 gal
hours = 4.72 ÷ gal per hour
Champion 3500 W: 4.72 ÷ 10.5 h = 0.45 gal/h
The 20 lb on the tank is the propane it holds when filled, so no further 80% comes off. That is close to SlashGear, which says "A 20-pound tank is only about 4.6 gallons of liquid propane, assuming you follow the 80% fill rule", and to Bluetti's 4.7 gallons. The 4.24 pounds per gallon is the "Lb. per. gal." row of the Engineering Toolbox LPG table.
What makers and guides publish
| Generator | Published | Hours on 20 lb |
|---|---|---|
| Champion 3500 W dual fuel (3,150 W on propane) | 10.5 h at 50% load | 10.5 h |
| Generac 7.5 kW | 0.87 gal/h half, 1.42 gal/h full | 5.4 h half, 3.3 h full |
| 2,000 to 3,500 W class (Bluetti) | 0.5 to 0.8 gal/h at 50% | 6 to 9 h |
| 5,000 to 7,000 W class (Bluetti) | 1.2 to 1.9 gal/h at 50% | 2.5 to 4 h |
- Champion's 3500W page (model 200965): "will run for 10.5 hours at 50% load when using a 20-pound propane tank", "3950 starting watts and 3150 running watts".
- SlashGear gives the Generac 7.5 kW as 0.87 gallons per hour at 50% load and 1.42 at full load, and works out about 5.3 and 3.2 hours with its 4.6 gallons. With 4.72 gallons here the hours are 5.4 and 3.3.
Why the numbers vary
Half load on the Champion 3500 W is about 1,575 W of output for 0.45 gal/h, or 0.29 gallons per kWh. Bluetti's 0.5 to 0.8 gal/h for the same size class works out higher, so guides for the same size disagree by 10 to 80 percent. I have no gallons-per-kWh figure that holds for every model, so use the table above with the number from your own spec sheet.
A tank that is not full runs in proportion: half full, half the hours. Run a generator outdoors, well away from windows and doors.
Related pages
For a grill or a patio heater, see how long a 20 lb propane tank lasts. For longer outages on a bigger cylinder, a 100 lb cylinder holds five times the gallons. A tank with a gauge is on the propane tank percentage chart.