Having recently witnessed my fellow eTool director Alex Bruce engaged in another animated discussion (ie. argument) over the relative benefits of six star ratings for most new homes in Australia as of 1st May 2012, I thought I would contribute to the debate with some of my own thoughtsA bit of background for those not familiar with the topic of discussion. NatHERS is the rating scheme currently legislated in Australia for encouraging all new homes to be more energy efficient. Rated out of a maximum ten stars, there is now a minimum requirement of six stars for most new homes throughout Australia, and depending on the state or territory, a minimum five star rating for renovations and additions.

Now, it’s really important to note that NatHERS only measures and rates the thermal control of a home – that is, the likely amount of energy that is required to heat and cool a home so that its occupants can be warm in winter, and not too hot in summer. Depending on which dataset you are looking at, thermal control makes up around 23% of the total energy use of a typical Australian household.

The theory goes like this – the more stars a home receives for its NatHERS rating, the less energy that will be required to heat and cool that home in order to maintain thermal comfort – a ten star home could theoretically use almost no energy at all to stay comfortable. However, this depends on which climate zone your home is situated in, as the star bands change for each of the 69 climate zones currently used as part of the rating scheme.

It’s also very important to note that NatHERS is not (and was never intended to be) a rating tool used to measure actual energy use in the home – it estimates heating and cooling loads for a home based on a number of assumptions about how we occupy them eg. such as opening windows to improve cross ventilation, occupying bedrooms primarily at night, living spaces during the day etc. It also doesn’t take into account how efficient an appliance may be at heating or cooling, nor will it account for a person’s individual behaviour eg. leaving a heating/cooling appliance turned on even if it’s not required to achieve thermal comfort.

That said, it is possible to extrapolate some numbers from the NatHERS star rating bands to determine how much it would cost to keep a home thermally comfortable. In no way is the following graph intended to represent actual costs; it should be considered only as a relative guide between the different star ratings:

Potential heating/cooling savings by star rating in Perth

Potential heating/cooling savings by star rating in Perth

The graph is also based on the following assumptions:

  • I’ve used the star rating thresholds for Perth (remember, these change for each of the 69 climate zones around Australia)
  • Have assumed a house size of 300 square metres
  • The heating/cooling appliance has a coefficient of performance (COP) of 1.0 – that is, for each unit of energy that needs to be added or removed from the building to maintain thermal comfort, the appliance doing this work will use one unit of electricity (in reality, a really efficient reverse-cycle air conditioner will have a COP of 3.0-4.0 ie. be 3-4 times more efficient)
  • The cost of one unit of electricity is around 25 cents (BTW 1.0 unit of electricity = 1kWh = 3.6MJ)
  • Doesn’t take into account the actual behaviour of the building occupant (eg. assumes heating/cooling appliance will be operated only when necessary)

As you can see, and would expect, the likely cost of heating/cooling a home decreases as the star rating increases. However, as you can also see, the potential saving is inversely exponential – that is, the amount of money you can potentially save decreases as the star rating increases.

I think this is a very important point to make. For example, consider the threshold between moving from 5 to 6 stars – there’s a potential saving of around $400.00 a year in heating and cooling costs, which I would argue is a saving worth making (I can think of a couple of things I would rather spend that kind of money on). I would also argue that it is a comparatively small cost increase to build a six star home as opposed to a five star home – some major builders are even doing this already without passing on the cost increase to their customers. Thus, the payback period is reasonably achievable.

However, now consider the jump from 9 to 10 star. There’s a potential saving of around $271 per year by doing this, which would mean a heating/cooling bill of less than $100.00 a year in Perth. However, if there is a substantial increase in the building cost in order to achieve this rating, it could be argued that the extra cost is difficult to justify. Depending on the design and climate, it may be better to aim for a lower rating that can be achieved for a more reasonable budget, and to then spend the extra money on solar PV to offset carbon emissions, or buy a more energy efficient fridge or TV (remember NatHERS doesn’t consider energy used by appliances, which by itself can represent around 15% of total residential energy use). Nor have we considered other environmental initiatives separate to energy efficiency, such as installing a rainwater tank or planting natives, or a waterwise garden.

Now don’t get me wrong – in no way should this post be misconstrued to suggest that I think star ratings are a bad idea (knowing my luck though, someone probably will). I sincerely believe NatHERS is a valuable rating tool that assists building design professionals to design more energy efficient homes, and that it is important to do so. However, if we get fixated just on winning more stars and saving dollars, then we’ve failed to comprehend the bigger sustainability picture. There are other effective ways we can – and should – promote building sustainability, such as making our homes more naturally comfortable and enjoyable, improving the design quality of the built environment, leading healthier lifestyles – these are just a few.

I also think it’s important that we remember that NatHERS only focuses on one part of the sustainability spectrum, and there are additional areas equally worthy of our attention, such as choosing energy efficient appliances, how we live in our homes, health impacts of building materials, reducing construction waste and embodied energy impacts, water harvesting and re-use – the list goes on. I don’t believe this diversity confuses the sustainability debate, but goes to highlight there are many things we can be doing to have a positive, beneficial impact on the environment, and that do not necessarily compromise our lifestyles, but actually enhance them.

Correction: in some parts of Australia, it is still possible for new buildings to comply without achieving a six star rating, for example in some parts of the Northern Territory and Tasmania, where state-based variations allow for a lower star rating. In NSW, the BASIX rating scheme is used in lieu of the NatHERS scheme – thanks to Taryn Cox for pointing this out.

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