Megajoules MJ – Megajoules to Kilojoules kj – Kilowatts kw
Megajoule is a unit of energy typically represented by the symbol “MJ”. MJ/hr indicates gas consumption of the appliance, not the heat output.
The prefix “mega” infers one million and megajoule equals 1,000,000 joules, an SI unit of energy with the plural “megajoules”.
What is a megajoule and why does it matter when I buy a gas heater?
What size gas heater should I buy?
Many people think they know but you might be surprised by the real answer…
What is a MJ – Megajoule?
♦ MJ is the symbol for Megajoule, which is a unit of measure relating to energy.
♦ Megajoules are based on a joule, which is the standard unit of energy in the International System of Units.
♦ As “mega” is the prefix for one million, a MJ equals one million (1,000,000) joules.
♦ The energy content of LPG and natural gas are both measured in megajoules.
♦ In the context of a gas appliance, MJ indicates the consumption of gas per hour – MJ/hr.
♦ MJ is sometimes incorrectly referred to as the heat output of a gas appliance. It is not.
♦ Kilowatt (kW) is the correct unit of measure for expressing heat output.
♦ How is it pronounced? Say it as if it were spelled “mega jewel” or “mega jewels”, not “mega jowl”.
Keep reading for the details on all of the above…
Kilojoules to Megajoules & Megajoules to Kilojoules
How many kilojoules in a megajoule? There are 1000 kilojoules in a megajoule.
What about megajoules to kilojoules? There is 0.001 MJ in a kilojoule.
A MJ is NOT Heat Output
I recently watched a product video produced by a major Australian retailer.
In it, a “Product Expert” reviews a Rinnai gas heater.
The problem was that the presenter lost me in the second sentence when he said: “That’s 13MJ of heat.”
This is an incorrect statement and perpetuates a common misconception that MJ represents heater output, as opposed to gas input.
Gas Consumption is in “MJ/hr” — Not Just “MJ”
Megajoules per Hour (MJ/hr) ratings on appliances actually indicate the gas consumption of the appliance, not the heat output.
On the other hand, heat output must also take into account the relative energy efficiency ratings of the models being compared.
The MJ/hr gas consumption rate is often erroneously expressed as just “MJ”.
This is how MJ is frequently used in reference to gas appliances.
Heat Output = Kilowatts or kW
Kilowatts (kW) is how gas heater output is measured. 3.6 MJ of input equals 1kW of output at 100% efficiency.
Conversely, 1 MJ = 0.278 kW.
However, no gas appliance is 100% efficient.
That is why we have Star Ratings, so we can compare relative efficiency between models.
Appliance output is often expressed as just kW.
Gas Heating
Gas heating is the heating of a room or outdoor area using a gas space heater fuelled by natural gas, LPG (propane or butane) or biogas.
Indoor gas heater types include unflued portable gas heaters, flued gas heaters, radiant gas heaters, convector gas heaters, radiant-convectors, and gas fireplaces.
Gas Heater
A gas heater is a space heater used for room heating or outdoor area heating fuelled by natural gas (including CNG and LNG) or LPG (propane, butane or a mixture of the two).
The two general types of home gas heaters are flued gas heaters (vented) and unflued gas heaters (unvented).
Energy Efficiency – Star Ratings
Star ratings were developed to provide consumers with an easy way of comparing the energy efficiency of different models.
All States now require both LPG and Natural Gas heaters to be tested and certified, under AS4553, by independent testing organisations, such as the AGA and SAI Global.
The Star Rating is based on net heater efficiency.
Net heater efficiency is calculated on the basis of a combination of efficiency measures and takes into account all gas and electrical inputs.
It basically compares the amount of raw energy input consumed by the heater in comparison to the heat energy output provided.
This is calculated as a percentage and then translated into the corresponding Star Rating.
Please see Energy Efficiency & Star Ratings for Gas Heaters
What Size Gas Heater Do I Need?
What to Consider
If you buy too small a heater, it will not be able to adequately heat the intended area.Heating Area Volume & Climate Zone
Very Cold Zone: 1kW output required for each 8.5m²
Adjustment Factors
Gas Heater Sizing Example #1:
Gas Heater Sizing Example #2:
Use kW not MJ
Energy Efficiency Matters – Star Ratings
Minimum Room Sizing
Manufacturer Sizing Charts
New Residential LPG customer?
New Business LPG customer?
Existing ELGAS customer?
Eric Hahn has over a decade of experience in the LPG industry and has become a key figure in shaping how LPG is marketed and communicated. As the creator of one of the world’s first LPG blogs, Eric has authored over 200 technical posts, helping educate and inform the industry and consumers.
Eric has served on multiple committees for Gas Energy Australia and the World LPG Association (WLPGA), contributing to important discussions and advancements within the global LPG sector. He has presented at WLPGA Annual Conferences, sharing his expertise as a speaker, panellist, and panel chair.
- Outdoor Heater: Outdoor Gas Heaters – Patio Heater Guide - September 22, 2024
- LPG Meaning – LPG Means: What Does LPG Stand For - August 31, 2024
- What Size & How Many Gas Bottles Do I Need? - August 29, 2024
Steve Reynolds
Technical Consultant
Steve Reynolds is a leading expert in the LPG industry with over 22 years of experience. As part of the national management team at ELGAS, Steve ensures the safe and efficient storage, handling, and transportation of LPG. He serves as the lead investigator for incidents and collaborates with authorities on industry developments.
Steve is a technical advisor to Standards Australia and Gas Energy Australia (GEA), and an active member of the World LPG Association (WLPGA), contributing to global standards and technical reviews. He holds a BSc. (Hons) in Industrial Chemistry from UNSW and has held senior safety and technical roles at ELGAS, making him a trusted authority in LPG safety and standards.