How long a propane tank lasts
The run time of a tank depends on how much energy it holds and how fast your appliance burns it.
- Energy in the tank = pounds of propane × 21,548 BTU per pound (about 91,500 BTU per gallon)
- Burn rate = appliance rating (BTU/h) × average burner setting
- Run time = energy in the tank ÷ burn rate
Appliances list their maximum rating, but you rarely run all burners on high. A grill at medium heat uses roughly half its rated BTU.
Worked example
You have a full 20 lb cylinder and a 36,000 BTU/h gas grill used at 50% for 3 hours a week, with propane at $3.50 a gallon.
- Energy in the tank: 20 × 21,548 = 430,960 BTU, or 4.7 gallons.
- Burn rate: 36,000 × 50% = 18,000 BTU/h.
- Run time: 430,960 ÷ 18,000 = 23.9 hours.
- At 3 hours a week, the tank lasts about 8 weeks.
- Cost: about $0.69 per hour of grilling.
Common tank sizes
| Tank | Propane when full | Energy |
|---|---|---|
| 20 lb grill cylinder | 20 lb (4.7 gal) | 431,000 BTU |
| 30 lb cylinder | 30 lb (7.1 gal) | 646,000 BTU |
| 100 lb cylinder | 100 lb (23.6 gal) | 2.2 million BTU |
| 250 gal tank | 200 gal usable | 18.3 million BTU |
| 500 gal tank | 400 gal usable | 36.5 million BTU |
Large tanks are filled to only 80% of their capacity, leaving room for the liquid to expand as it warms. Many exchange cylinders are filled with 15 lb instead of 20 lb, so check the label and adjust the fill level.
Typical appliance ratings
| Appliance | BTU per hour |
|---|---|
| Portable camping stove | 10,000 to 20,000 |
| Gas grill | 30,000 to 60,000 |
| Patio heater | 40,000 to 48,000 |
| Fire pit | 30,000 to 65,000 |
| Tankless water heater | 140,000 to 200,000 |
| Home furnace | 60,000 to 100,000 |
Checking the level
Weigh the cylinder on a bathroom scale and subtract the tare weight (TW) stamped on the collar: a 20 lb tank weighs about 17 lb empty. Large tanks have a gauge showing the percentage of capacity. Another trick: pour warm water down the side of the cylinder and feel for the cool line where the liquid starts.
Safety
Store cylinders outdoors, upright and away from heat, never in a garage, basement or car. Check connections with soapy water: bubbles mean a leak. Never use a grill or patio heater indoors or in a tent, because of the risk of carbon monoxide poisoning.