Liquefied petroleum (LP) gas is propane mixed with butane and placed under pressure to liquefy it. The energy content of LP gas is 91,000 BTUs per gallon. Thus, a 40,000 BTU BBQ grill burner using propane at 80 percent efficiency uses (40,000 / 91,000) /0.80 = 0.55 gallons of propane per hour.
Don't confuse device efficiency with burner efficiency. Most gas burners are very efficient. It is the how well the device uses the heat provided by burners, and how much heat goes up the chimney, that determines device efficiency.
Gas burners consume fuel based on their BTU output. Natural and propane gas have specific levels of energy contained within them. One cubic-foot of natural gas contains 1075 BTUs of energy. However, gas burners are not 100-percent efficient. If you know the burner efficiency, you can calculate the exact fuel consumption per the output of the burner. If you do not, you will have to use an approximation. LP gas and natural gas contain different amounts of energy, so the conversion factor is different.
Approximate Gas Consumption
Find the burner output rating from the user documentation or the appliance sticker such as that found on a water heater. The output rating will be listed in BTU (such as 40,000 BTU). BTU stands for British Thermal Units.
Determine the fuel being used and select the conversion factor. Natural gas burners use an approximate conversion factor of 1,000. LP or Propane uses an approximate conversion factor of 2,516. This assumes the burner is less than 100-percent efficient when it converts gas to heat energy, which is different from the efficiency of the device using the heat. A furnace burner might 93-percent efficient, but the furnace itself may be only 80-percent efficient at using the heat.
Divide the BTU output by the approximate conversion factor. A natural gas burner rated at 40,000 BTUs per hour divided by 1,000 BTUs per cubic foot uses approximately 40 cubic feet of natural gas in one hour.
Exact Gas Consumption
Multiply the burner efficiency by the energy content of the gas being used. Natural gas contains 1,075 BTUs per cubic foot; propane contains 2570 BTUs per cubic foot.
Determine the burner output in BTUs per hour from the appliance sticker or user documentation. The output will be in BTU (for example, 40,000 BTU).
Divide the BTUs per hour by the fuel energy content. Our burner with the 40,000 BTU per hour output divided by 1,075 equals 37.21 cubic feet of gas per hour. This is consumption at 100-percent efficiency
Adjust for the burner efficiency by dividing consumption at 100 percent efficiency by the burner efficiency rating. For a burner that is 95-percent efficient, divide by 0.95. In the example: 40,000 BTU divided by 1,075 = 37.21. Adjusting for burner efficiency: 37.21 divided by 0.95 = 39.17 cubic feet of natural gas consumed in one hour.
Michael Logan is a writer, editor and web page designer. His professional background includes electrical, computer and test engineering, real estate investment, network engineering and management, programming and remodeling company owner. Logan has been writing professionally since he was first published in "Test & Measurement World" in 1989.