How NatHERS star ratings are calculated

When calculating a star rating, NatHERS Accredited Software models expected indoor temperatures based on data specific to that dwelling. This includes information on design and construction, climate and common patterns of household use. The software tools do not take into account electrical appliances in the dwelling, apart from the air flow impacts from ceiling fans.

Using the software user interface, a NatHERS Accredited Assessor enters data to develop a simulation model of the house or apartment, including:

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

Tape measure plans

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The software will then model how much cooling or heating occupants may need to stay comfortable during a typical year. It does this by firstly simulating how the local climate heats and cools the house every hour of every day of the year.

Once internal temperatures fall outside a comfortable range, the software models the need for heating and cooling on the assumption that the occupants of the house will firstly open and close windows and any blinds or awnings to restore comfortable internal temperatures (when it makes sense to do so) before using any heating or cooling system. The total estimated annual heating and cooling requirements are then converted to a star rating out of 10 stars.

To enable comparison across different homes each software package uses the same:

  • climate zones
  • weather files
  • occupancy settings for a 'standard family'
  • internal heat loads
  • star rating scale for the home's location

NatHERS Climate Zones and Weather Files

NatHERS divides Australia into 69 climate zones. This allows the software to appropriately consider the effects of Australia’s diverse climate when modelling the heating and cooling requirements of a home.

Weather files have been compiled from Bureau of Meteorology records for each NatHERS climate zone and include information on air temperature, humidity, solar radiation and wind speed and direction. The weather file for each climate zone, called the Reference Meteorological Year (RMY), is compiled from at least 25 years of Bureau records.

When conducting a NatHERS rating, the software uses the weather files to assess the impact of weather on the building’s internal temperature. This impact is calculated for every hour in a full 12 month period.

Read more about climate zones and weather files.

Internal Heat loads

To allow fair comparison of different homes, modelling using NatHERS software is based on consistent assumptions about how occupants would use a home, including which rooms are typically used at different times of the day. When a room is being used, the software takes into account the latent heat energy of moisture in the air (humidity), heat energy generated by occupants themselves, and heat generated from cooking, lighting, and electrical appliances. These factors combined are called an ‘internal sensible and latent heat load’. This heat load impacts on internal temperatures and the need for artificial heating and cooling.

Modelling using NatHERS software is also based on consistent assumptions about the internal temperatures that would be comfortable for occupants. These thermostat settings differ by climate zone, as people generally adapt to their local conditions.

Read more about Internal heat loads.

Further details about how heat loads are determined are included in the NatHERS Software Accreditation Protocol [PDF 681 KB].

Home size

The software calculates the family's annual combined cooling and heating needs, which are compared to the size of the home. Area correction factors are applied to allow fair comparison of different sized buildings in different locations.

Read more about Area correction factor.