Things You'll Need
Appliance Btu rating
It's important to have an idea of how much propane an appliance uses so that you don't run out at an inopportune time. An energy-use formula fills the bill. Each propane tank size has an approximate capacity measured in British thermal units. You can use this capacity in conjunction with your appliance's Btu rating to determine how long a full propane tank will power any given appliance. An appliance's Btu rating assumes that the device is running at 100 percent, such as a gas grill running with all burners set on "high."
Measure your propane tank's height to help determine its size. The height should be measured with the tank standing on its bottom, from the floor to the base of the propane tank collar. The propane tank collar is the raised piece surrounding the tank valve.
Measure the circumference of the tank at the widest point by wrapping your measuring tape around the tank. Divide the circumference by pi, approximated at 3.14, to get the diameter.
Determine the pound capacity of your tank with the measurements taken in Steps 1 and 2. A 20-pound tank is 18 inches high and has a diameter of 12.5 inches. A 30-pound tank is 24 inches high and has a diameter of 12.5 inches. A 40-pound tank is 29 inches high and has a diameter of 12.5 inches. And a 100-pound tank is 48 inches high and has a diameter of 14.5 inches.
Determine your tank's Btu capacity when full using the pound capacity. A 20-pound propane tank has a 430,270-Btu capacity; a 30-pound tank, 649,980 Btus; a 40-pound tank, 860,542 Btus; and a 100-pound tank, 2.16 million Btus.
Locate the appliance's Btu rating. This can be found either on the appliance itself or in the owner's manual.
Divide your propane tank's Btu capacity by the appliance's Btu rating to determine how many hours your propane tank will power the appliance at 100 percent.
A 20-pound tank is used to power gas grills. Because 1 pound of propane will power a gas grill for 30 minutes, a full 20-pound propane tank will power a gas grill for 10 hours.
Never use a larger propane tank than your appliance manual recommends.