LPG (propane) gas consumption is based on the burner size or sizes of the appliance, as some have more than one burner. All gas appliances are rated by the manufacturers for propane gas consumption. To work out how many hours burn in an LPG gas bottle, divide the energy (MJ) by the total MJ input of your appliance (MJ /h). They are expressed in either MJ/hr or BTU/hr, depending on which country you live in. The propane gas consumption ratings are typically found in the product specifications.
How to Calculate the LPG Consumption per Hour
1. Determine the MJ or BTU rating of the appliance, which is the gas energy consumption per hour, NOT the heat output. Find it in the specifications or owner’s manual.
2. Look up the energy content of the fuel you are using in the unit of measure you want to use. For example:
1 L = 25 MJ of energy from LPG
1 kg = 49 MJ of energy from LPG
1 Gal = 91,502 BTU of energy from propane
1 lb = 21,594 BTU of energy from propane
3. Divide the energy content by the appliance consumption rating. For example:
A gallon of propane contains 91,502 BTU of heat energy whilst the BTU rating for appliances is in BTU per hour, based on burner size.
Dividing 91,502 by the BTU per hour rating gives you the number of hours that one gallon of propane will last.
A 23,700 BTU heater will last 3.86 hours (91,502 ÷ 23,700 = 3.86)
Example #1: A 25MJ Heater
If you have a heater that consumes 25MJ/hr, to calculate the LPG consumption per hour, just divide 25 by the values in the MJ column of the consumption conversion chart.
25÷25 = 1 litre per hour (LPG MJ per litre)
25÷49 = 0.51kg per hour (LPG MJ per kg)
25÷96.5 = 0.259 gallons per hour (LPG MJ per Gallon)
25÷22.8 = 1.1 pounds per hour (LPG MJ per Pound)
Example #2: A 14,000 BTU Heater
If you have a heater that consumes 14,000 BTU/hr, to calculate the LPG consumption per hour, just divide 14,000 by the values in the MJ column of the consumption conversion chart.
14,000÷23700 = 0.59 litres per hour
14,000÷46,452 = 0.3kg per hour
14,000÷91,502 = 0.153 gallons per hour
14,000÷21,594 = 0.648 pounds per hour
If you have an appliance with more than one burner, like a cooktop, just add the burner ratings together before dividing by the chart values.
What is a Megajoule or MJ?
Most people know that gas appliances are rated in Megajoules or MJ/hour. However, many people believe that this is a measure of output. In reality, it is a measure of the required gas input. Output is typically measured in kilowatts. The MJ inputs and kW outputs are directly related but are affected by the efficiency of the appliance.
What is a BTU?
A BTU – British Thermal Unit – is an old non-metric unit of measure for heat. One BTU will raise the temperature of one pound of water by 1°F. BTU is used in the USA instead of MJ. Most of the rest of the world uses MJ. 1MJ equals 948BTU, so to get an equivalent number of Megajoules, just divide the BTU rating by 948.
As mentioned above, the gas consumption rating is not the same as the heat output rating.Output is typically measured in kilowatts. For reference, 1kW = 3.6MJ. The MJ inputs and kW outputs are directly related but are affected by the efficiency of the appliance. For example, a 25MJ heater with a 5.8 Star energy rating has an output of 6.2kW. Now, if you do the maths, dividing 25MJ ÷ 3.6MJ, you would expect that the output to be 6.94kW, not 6.2kW. The difference is that, at 5.8 Stars, the heater is 89.3% efficient. No gas appliance is 100% efficient, which is why we have the gas appliance Star rating system to judge the relative performance of different models.
Appliances aren’t always used on the highest setting. For example, the popular Rinnai Avenger input is 25MJ on high but only 8.5MJ when set on low. Hot water heaters will also automatically modulate the burner down, once the desired temperature has been achieved. As a result, the consumption will be less than what would occur at the maximum appliance input rating. Some manufacturers provide the consumption specifications for the lower settings, so you can still do your calculations.
Final Thoughts to Calculate the LPG Consumption per Hour
You can calculate the LPG consumption per hour to determine how long a gas bottle will last. For example, if you have a 45kg LPG gas bottle or a 100lb propane tank, you just divide by the kg/hr or pound/hr calculated values to determine how long the gas bottle will last. For those who can’t be bothered to calculate the LPG consumption per hour, here is a quick reference chart with examples:
Propane Usage Chart for 45KG Gas Bottle
This propane usage chart shows how long a 45kg gas bottles will last when fuelling a 15MJ heaters, 25MJ heater, 125MJ hot water system, 199MJ hot water system and a 9MJ cooktop burner.