Heat pump star ratings

by Richard Keech


There may be some confusion about performance ratings for heat-pump air conditioning units.  Here are a few thoughts and resources that may clarify things.

IMG_2137 lores

MEPS and Star ratings

In Australia the federal government regulates appliance energy efficiency.  There are, broadly, two main ways that appliances are regulated – MEPS and energy rating labels.  MEPS = minimum energy performance standard.  Appliances subject to MEPS are regulated such that it is illegal to sell them in Australia if they don’t meet a minimum performance level.  This is different from the star rating on the energy rating label shown at point of sale.

Appliances subject to energy ratings labels are required to disclose the energy performance.  Depending on regulation, a given type of appliance, might be subject to either MEPS or star ratings, or both.


The main measure of the energy performance of any heat pump is the ratio of the a) rate of heat energy delivered or removed to b) the electrical input power.  For heating this measure is called the coefficient of performance (CoP). For cooling it’s called the energy efficiency ratio (EER).

For a given unit, COP and EER vary depending on many factors, including especially outside air temperature.  So to make it easy for rating purposes there’s a standardised measure corresponding to the average performance over an entire year.  This is the annual CoP (ACOP) and annual EER (AEER).

Standby power and COP

In Australian standards, the calculation of AEER and ACOP take into account the standby power used based on an assumed 2000 hours per year of operation.  So the standby energy for the remaining 6760 hours per year is added into the denominator of the calculation so that high standby power will lower AEER and ACOP (all other things being equal).

Applying this to heat pumps

Most, but not all heat-pump aircon units in Australia are subject to both MEPS and star ratings.  Units excluded, for example, include three-phase systems, ducted systems,  and systems rated at over 65kW.

So your basic single-phase, non-ducted, domestic aircon unit is subject to both mandatory MEPS requirements, and star ratings.

Aircon MEPS

The mandatory minimum aircon performance for ‘unitary’, i.e. single split systems with a capacity of less than 20kW is an annual CoP or EER of 3.1.   The regulation is based on Australian Standard AS/NZS 3823.2, last updated in 2013 (title: “Performance of electrical appliances – Air conditioners and heat pumps. Part 2: Energy labelling and minimum energy performance standards (MEPS) requirements”).

Multi-split aircon units have a slightly higher MEPS level.  For units <4kW capacity the level is 3.66.  From 4kW to 10kW the MEPS level is 3.11.

Aircon star ratings

The calculation of the star rating is also defined in AS/NZS 3823.2.   The star rating index (SRI) is calculated separately for heating and cooling, and purely a function of ACOP or AEER.  The formula is:

Cooling:   SRI = 2*AEER – 4.5

Heating:  SRI = 2*ACOP – 4.5

When published, the star rating is rounded down to the half-star level.

So the formula for star rating is the same for both heating and cooling.

The star rating = CoP  at a CoP of 4.5.

Minimum star rating. The MEPS level is for ACOP of 3.1 as mentioned above.  This corresponds to a star rating of 1.7 (or rounded down: 1.5).  So it is illegal to sell an aircon below this level.

A change in CoP of 1 corresponds to a change in star rating of 2.

A hypothetical CoP of 2.25 corresponds to a star rating of 0.

Portable units

It’s interesting to note that currently portable aircon units are not subject to energy performance regulations.  Generally portable aircon units are understood to be very inefficient.  It seems obvious that these units should be subject to mandatory energy performance standards.

Further reading