What’s in a Star?

by Dr Paul Bannister | Nov 01, 2016
The NABERS language of stars has become so ubiquitous that most people work regularly with the scheme without actually having a strong appreciation of what the stars actually mean in terms of carbon dioxide emissions. So here is a short primer on the NABERS star scale for office base building ratings. I have based the discussion on the office base building energy rating scale for NSW; the rating scales for other states show identical or very similar behaviours.

The NABERS star rating scale originally ran from 1 star to 5 stars where 2.5 stars was set to the state average and 5 stars was set, via simulation, as a benchmark for “as good as it could get” for a conventional building.  The scale was set as a linear relationship between emissions per m² (normalised for hours of use) and the star rating.  The original scale was set as a nationwide benchmark in 1999; NSW, ACT and SA still use this original scale.  The other states and the Northern Territory updated their rating scales – using the same principles – based on state-specific data collection – over the period 2000-2005.  Victoria undertook a further small modification to their scale around 2010. 

In 2011, the rating scale was extended to 6 stars using the simple expedient that 6 stars is half the emissions of 5 stars and that thus – by extrapolation – a building with zero emissions would have a theoretical rating of 7 stars, if such a rating were possible. The Figure below illustrates the base building energy for offices rating scale for NSW.  You can see that from 1-5 stars there is a constant slope of 32kg/m² per star.  Above 5 stars, this changes to 35.5kg/m².  For other states, the kg/m² steps are different based on local greenhouse emissions coefficients, but the effective kWh/m² steps are reasonably similar from state to state.

What's in a star

The linear scale has the advantage that the emissions saved by going up a star are essentially constant.  This is intended to focus building owners on the fact that it is just as worthy – and typically cheaper – to upgrade poorly performing assets as it is to upgrade better performing buildings by the same amount.   

However, this does have the consequence that the percentage improvement from star to star increases as the base star rating gets higher.  Thus taking a building from 1 to 1.5 stars requires only an 8% improvement while improving from 5 to 5.5 stars requires a 25% improvement.  The amazing thing is that buildings have been achieving the improvement necessary to get from 5 to 5.5, and higher.  Indeed, Energy Action has recently succeeded in tuning 747 Collins St to 6 stars (33% better than 5.5 stars) just through operational measures and tuning.

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