What Does a NatHERS Star Rating Mean

What is the purpose of a NatHERS Star Rating?

The NatHERS (Nationwide House Energy Rating Scheme) star rating is a measure designed to compare the energy efficiency of home designs across Australia, taking into account the country's diverse climate zones. Australia is divided into 69 climate zones, ranging from cold regions like Tasmania to tropical areas such as Cairns and Darwin. Given these vastly different climates, a single, universal energy efficiency measure wouldn't be fair or accurate.

To address this, the NatHERS system uses a table called the "Star-bands," which assigns a range of energy usages to star ratings from 0.5 stars to 10 stars, in increments of 0.5 stars. (Increments of 0.1 stars on the certificate, this is interpolated between the given star-band values). These energy values are adjusted so that homes with similar efficiencies in different climate zones receive comparable star ratings. This adjustment ensures that you can compare the energy efficiency of homes in different climate zones independently of the local climate conditions.

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 star 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.

The Chenath Engine, developed by CSIRO, underpins the NatHERS rating system. 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 there are criticisms regarding the assumptions used by the Chenath Engine, the consistency of these assumptions means that every home is measured by the same standard. This uniformity ensures a consistent rating system across Australia, even if the absolute values might differ slightly when compared to other rating schemes like the Passive House standard.

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