How the calculation works
Propane is sold by volume but appliances are rated in BTU per hour, so the conversion runs through the energy content of the fuel.
Two practical notes on tank size. Tanks are filled to roughly 80% of their nominal capacity to leave room for expansion, so a "20 pound" barbecue bottle holds about 4.7 usable gallons rather than five and a half. And appliances rarely run at full rate continuously; thermostatically controlled heaters and water heaters cycle, so real consumption is often well under the calculated maximum.
Worked example
A 30,000 BTU heater running four hours a day from a 20 gallon tank:
The same tank serving only a propane fridge at 1,500 BTU per hour running continuously would last several months, which shows how sharply heating dominates any propane budget.
Why off-grid homes use propane at all
Because heat is expensive to make with electricity. Running that 30,000 BTU heater electrically would need roughly 8.8 kW, more than most off-grid inverters can supply and far more than a battery bank can sustain for four hours. Propane, wood and solar thermal handle heat; the electrical system handles everything else. Splitting the job that way is what keeps off-grid systems affordable.
The trade is dependence on deliveries and the cost per unit of energy, which is why many designs use propane for cooking and water heating, wood for space heat, and reserve electricity for lighting, refrigeration, pumping and electronics.
Common mistakes
- Assuming continuous operation. Thermostatic appliances cycle. Treat the calculated figure as the worst case and expect real consumption to be lower.
- Using nominal tank size. Tanks fill to about 80%. Plan refills against usable volume, not the label.
- Forgetting cold weather regulator issues. In hard frost, small bottles can struggle to vaporise fast enough for a large appliance. Larger tanks provide more vaporisation surface.
- Running the tank to empty. Refill at around 20% remaining. Emptying the tank can pull oily residue into regulators, and running out mid winter is more than an inconvenience.
Frequently asked questions
How long does a 20 pound propane bottle last?
Roughly 4.7 usable gallons, so about 430,000 BTU. On a 7,000 BTU cooktop used one hour a day that is over sixty days; on a 30,000 BTU heater running four hours a day it is about three and a half days.
Is propane cheaper than electric heating off-grid?
Almost always, once you count the solar and battery capacity electric heat would require. Grid-connected the comparison is closer and depends entirely on local fuel and electricity prices.
How do I know how much is left in the tank?
Weigh it. Tank tare weight is stamped on the collar; subtract it from the current weight and divide by about 4.2 pounds per gallon. Gauges on small bottles are approximate at best, and the hot water trick down the side only shows the liquid line roughly.