Propane Tank Duration for Generator
Pick your tank, your generator and the load, and see how many hours and days the propane lasts, running around the clock or 12 hours a day.
Propane tank duration
E06Scenario A
20 lb = 4.6 gal · 100 lb = 23.6 gal · fixed tanks 120, 250, 500, 1,000 gal.
Continuous runtime
Fixed tanks are filled to 80% (NFPA 58), so the usable fuel is 80% of the tank size.
- Usable gallons
- 200.0
- Propane use
- 2.10 gal/h
- Continuous runtime
- 95 h (4.0 days)
- Running 12 h a day
- 7.9 days
Estimate. Formula and sources are under “How it works”. Change any input; the result updates as you type.
Formula
How it works
- Usable propane = tank gallons × 0.8 for fixed tanks (100 gallons and up); a cylinder's listed propane capacity for 20 lb and 100 lb cylinders
- Gallons per hour = propane use at 50% load for that generator size × (load ÷ 0.5)^0.8
- Hours of continuous run = usable propane ÷ gallons per hour
- Days at 12 hours a day = hours ÷ 12
Fixed propane tanks are never filled to 100%: safety codes leave about 20% for the liquid to expand. A "250-gallon" tank holds about 200 gallons of propane when full, and less if it wasn't topped off before the storm.
Sources: 80% fill limit (NFPA 58); 20 lb cylinder ≈ 4.6 gal, 100 lb cylinder ≈ 23.6 gal of propane. Propane fuel use at 50% load from manufacturer spec sheets (Generac, Champion, Honda, Westinghouse).
Worked example
A worked example
(test-cases.json · E06 · case 2: 100 lb cylinder (23.6 gal), 7,500 W, 50% load)
A 100 lb propane cylinder holds about 23.6 gallons. A 7,500 W generator at half load burns 1.20 gal/h, so the cylinder lasts about 20 hours of continuous running, 0.8 days. Run it 12 hours a day and it lasts 1.6 days.
Questions
FAQ
How long will a 500 gallon propane tank run a generator?
A 500-gallon tank holds about 400 gallons when filled to 80%. A 22,000 W standby generator at half load burns about 2.10 gal/h, so that's roughly 190 hours, 7.9 days of continuous running, or 15.9 days at 12 hours a day.
How long does a 20 lb propane tank last on a generator?
Not long. A 20 lb cylinder holds about 4.6 gallons; a 5,500 W generator at half load burns about 0.90 gal/h, so it lasts about 5 hours. Keep several cylinders, or connect a 100 lb one.
Should I get a bigger propane tank?
Compare the result with the longest outage you plan for. A 250-gallon tank runs a 22,000 W unit at half load for about 95 hours, 4.0 days; a 500-gallon tank doubles that. Deliveries can stop during widespread storms, so plan on what's in the tank.
Why does the load matter so much?
Fuel use rises almost in step with load. Keeping big loads (central AC, electric water heater, dryer) off during an outage is the easiest way to add days to the tank.
Does cold weather change this?
Propane vaporizes more slowly in cold weather, which can limit what a small cylinder can deliver. This calculator doesn't model that; your propane supplier can size the tank for your climate.
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