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The NatHERS (Nationwide House Energy Rating Scheme) Star rating is designed to compare the energy efficiency of different home designs across Australia. In order to take into account Australia’s diverse climate, the country is divided into 69 different climate zones ranging from Tasmania to Cairns and Darwin with such variation between different climates in Australia.
What is the NatHERS Star rating based on?
The NatHERS Star rating is based on software simulation of the home using the climate data from these 69 climate zones, the home is modelled in specialised software, specifying the construction methods and the geometry of the home and the shading around the home. And then using a simulation engine, calculations are done for every hour of a year (8760 hours in a year) to determine the indoor air temperature of each room in the home and determine how much heating or cooling the home will need thought the year to be kept at a comfortable temperature.
The output of the NatHERS Star rating simulation is ultimately boiled down to a total energy used for one year. Because of the vast differences in Australia between climate zones, this value is converted to a “Star” rating using a table called the Star-Bands which assigns a Star Rating so that homes with similar efficiencies in different climate zones would receive similar star ratings. This ensures that the energy efficiency of homes is able to be compared regardless of the climate zone the dwelling is in.
What is the Star Rating Measuring?
In simple terms, the NatHERS star rating measures the energy per square meter (measured in Megajoules, MJ/m²) that a home will use for heating and cooling over one year. The NatHERS rating focuses exclusively on the energy required to maintain comfortable indoor temperatures and does not consider other factors such as lighting, hot water, water usage, cooking, or appliance usage. For a more comprehensive energy rating that includes these factors, the “Whole-of-Home” rating should be consulted.
How is a NatHERS Star rating Calculated?
The Chenath Engine, developed by CSIRO, underpins the NatHERS rating tools. It simulates the internal air temperature of each room in a home using weather data from the Bureau of Meteorology (BOM). By combining this temperature data with assumptions about room usage patterns, the Chenath Engine estimates the energy needed to heat or cool each room to a predetermined comfortable level.
Although the assumptions used by the Chenath engine are open to criticism, the consistency of these assumptions means that every home is measured to the same standard. This uniformity ensures that across all of Australia the NatHERS rating is consistent even though the absolute values might vary when compared with other housing rating standards, Passivehaus for example.
Conclusion
Even though the NatHERS Star rating system does not include various parts that whole of home assessments, it plays an important role in ensuring that there is a consistent yardstick that all homes in Australia are able to be measured against. Even though there may be arguments about the assumptions used by NatHERS and other rating systems, by having an agreed upon standard, we can ensure that all homes are compared equally against each other.