BTU, or British Thermal Unit is what the entire propane world revolves around. Surprisingly, the LP Gas industry is not solely dependent upon gallons or pounds of propane to make informed decisions. Industrial, commercial and residential customers alike are at the mercy of BTU's for their propane energy needs. A BTU is a unit of measurement, much like a gallon or pound.
It would be much easier for manufacturers of propane appliances and equipment to express gas usage in terms of gallons instead of BTU's because propane users buy their gas "by the gallon". After all, the appliances are using those same gallons purchased from the propane company, much like gallons of gas pumped into a car's gas tank. But when you buy a car, you don't ask about the gallons of gas the car will use, you would rather ask how many miles per gallon the car gets. If your vehicle needs involve hauling heavy loads, you would probably be interested in engine and/or towing power. Although a crude comparison, BTU's are in essence a measurement of consumption or deliverable power applicable to individual LP Gas appliances. While vehicles and engines are made to run on diesel, gasoline, propane or other fuels, the engine is rated by horsepower as a factor for explaining how much power the engine is capable of generating. In the gas appliance world, we equate BTU's to be somewhat of an "appliance horsepower" measurement.
Appliance BTU's are expressed through BTU input ratings and are based on a "per hour" basis. Whereas a furnace may be rated for 100,000 BTU, a furnace rated at 200,000 is capable of delivering twice the "horsepower" of the 100,000 BTU furnace. It's also capable of delivering 5 times the horsepower of a 40,000 BTU space heater. In the propane world, a 200,000 BTU furnace is not needed to heat a room when a 40,000 BTU heater will do the job. As you see, BTU ratings are used to appropriately match appliances for the required need and appropriate purpose.
BTU's are used by propane companies to determine total LP Gas appliance load. The total load is expressed in BTU's and represents the total propane demand on a system when all gas appliances are operating at full capacity. The BTU load help the propane company select an appropriate tank size for the installation as well as select pipe size and regulators so that the downstream appliances will work efficiently. For instance, a large home with several furnaces and water heaters will require many more BTU's than a one room cabin with a propane stove. The larger home has much more propane appliance "horsepower" than the small cabin. This is the sole reason that BTU's are the factor of measurement in the propane industry as opposed to using gallons for measurement.