A Megajoule or MJ Probably Isn't What You Think
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 MJ?
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.
So, 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.
How is it pronounced?
Say it as if it were spelled "mega jewel" or "mega jewels", not "mega jowl".
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 = kWh
Kilowatt Hour (kWh) is how gas heater output is measured. 3.6 MJ of input equals 1kWh of output at 100% efficiency.
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.
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.
What Size Gas Heater Do I Need?
What to Consider
Heating Area Volume & Climate Zone
Very Cold Zone: 1kW output required for each 8.5m²
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
Compare Indoor Gas Heater Prices
Now you can compare prices for all the major brands of indoor gas heaters including:
Rinnai, Everdure, Paloma, Braemar and Cannon
Click on your category of interest to compare prices:
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Comments, questions or feedback?
The information in this article is derived from various sources and is believed to be correct at the time of publication. However, the information may not be error free and may not be applicable in all circumstances.