Frequently asked questions
Here are answers to some of the questions commonly asked about NatHERS ratings.
Do all software packages give the same star rating for the same house?
Not always. The older first generation tools such as BERS, NatHERS and FirstRate used different methods to create the building description and they calculate their results in different ways. The star ratings they produce may differ slightly. First generation software tools are being phased out in 2007. All second generation tools are required to meet stringent requirements and will be accredited to the Scheme only if they demonstrate that results from a specified sample of buildings are consistent with those published by the National Administrator.
My house rated 5 stars with the original NatHERS software. Will it have the same rating under AccuRate?
Not necessarily. AccuRate and all the second generation tools simulate features of a dwelling that NatHERS software simply could not. The number of climate files has more than doubled and the assumed behaviour of the "standard family" has changed. These differences mean that ratings may change between first and second generation software tools.
I thought 5 stars was the highest rating that could be achieved. What is the meaning of 6 and 7 star ratings?
When the NatHERS Scheme began, 99 per cent of Australian houses fell below the 5 star standard. A 5 star scale seemed enough to cover the vast bulk of housing designs at the time. As energy efficiency levels have improved and better software is available to measure higher performance, the full 10 star scale (equivalent to zero conditioning energy) has been brought into play for the rating scheme.
A house energy rating is a very good guide to how much heating or cooling might be needed to keep your home comfortable. Your own heating and cooling bills will depend on how you run the house, whether you choose to install air conditioning and/or heating appliances and what types you select. The most efficient types use a fraction of the energy needed by the least efficient.
NatHERS Scheme ratings focus attention on the energy systems that are most affected by climate variability. Ratings do not consider the energy used by hot water systems or by major household appliances (such as refrigerators, stoves, ovens, dishwashers or washing machines), lights or the wide range of smaller appliances used in houses. These items often account for more than half of household energy bills and are important to the overall environmental impact of a dwelling, but the associated energy consumption is not greatly influenced by the intrinsic building design or the local climate.
Why aren't hot water systems or other appliances part of the star rating?
The hot water system and all the major appliances such as washing machines, dryers or refrigerators that may be supplied with a house usually have a much shorter working life than the house itself. A house energy rating is based on the parts of the house that are least likely to be replaced - roofs, walls, windows and floors. That way, the rating does not change each time a new hot water system is installed or the dishwasher replaced.