What is the NatHERS Star Rating?

Energy Efficiency Certification

The Nationwide House Energy Rating Scheme (NatHERS) was introduced in 2003 to improve the energy efficiency of homes. The Scheme allows for existing and yet to be built houses to be rated on their level of thermal comfort. The more stars a house is rated, the less likely cooling or heating will be required to stay comfortable inside. At the time of introduction only one percent of homes in Australia achieved six stars. The average house built in the 1990s was rated a one star however following the introduction of NatHERS, a well-designed house can now achieve a seven star rating.

What is taken into consideration?

Not everything inside a house is taken into account when calculating its star rating. The hot water systems, household appliances and lights are not considered therefore it is possible to calculate a house’s star rating before or after building. Ceiling fans are the only electrical appliances taken into account due to the change in air-flow they provide.

The rating is determined by:

  • layout of the house
  • construction of the roof, walls, windows and floor
  • orientation of windows and shading to the sun's path and breezes
  • the suitability of these items to the climate

Star Ratings

A range for star ratings is called a star band which is set for each climate zone. Australia is divided into 69 climate zones. Each climate zone has information on air temperature, humidity, solar radiation, wind speed and direction supplied by the Bureau of Meteorology. The Bureau’s weather files are compiled using at least 25 years of records.  

The star band takes into account the weather extremes of the climate zone and is based on the maximum energy consumption per unit area (MJ/m2). The maximum 10 star rating means the house will remain comfortable year round without any artificial heating or cooling.

Calculation of NatHERS Star Rating

NatHERS Accredited Software is used to model the expected indoor temperature using data for the house. The common patterns of the household are taken into account as well as design, construction materials and climate. A NatHERS Accredited Assessor enters data into the software for a simulation model of the house which includes:

  • size and function of rooms
  • size and specification of openings
  • building materials/windows/products
  • type of construction
  • dwelling orientation
  • location

The model simulates how the climate will cause the house to heat and cool every hour of each day of the year. For the hours the house is hotter or colder than the comfortable range, the software models the need for heating and cooling. It is assumed that people living in the house will open doors, windows, blinds and awnings to return the house to the comfortable range. Once this is not an effective method, heating and cooling will be applied. The model calculates the estimated annual heating and cooling requirements for an average year.      

The software also considers heat loads which is a consistent assumption of how a house is used in particular how the rooms are used at different times of the day, the heat generated by the people inside, cooking, lighting and electrical appliances. The heat load affects the internal temperatures. For more information about the process we undertake, see Energy Efficiency Certification.

If you would like a star rating for an existing or yet to be built house or apartment, contact Advanced Building Engineers NatHERS Assessor on 9382 8888 or email abewa@abewa.com.au.