Star ratings have been developed to provide consumers with an easy way of comparing the energy efficiency of different models. In this context, energy efficiency is defined as the “energy service per unit of energy consumption”. It is a simple and fair way of comparing the energy consumption of products that perform a similar task.

If you think of an air conditioner, for example, a model’s efficiency is the amount of cooling capacity (output) per unit of energy it consumes (input). A similar way of measuring the “energy efficiency” has been developed for all labelled appliances. Looking at the energy alone is not all that helpful, as a big appliance will nearly always use more energy than a small one, because it is doing more work. A measure of energy efficiency means that you can directly compare a greater range of products.

For most products, the standard star rating system has a minimum of 1 star and a maximum of 6, shown in half star increments. Various “algorithms” or equations have been developed to rate the least efficient products at around 1 star. Starting in April 2010 (in Australia), the star rating system has been updated for refrigerators and air conditioners to recognise super efficient products – eligible products can earn up to 10 stars.

If appliance energy efficiency in the market never changed, the stars could be adjusted so that the best products on the market now rated 6 stars. However, we know from experience that manufacturers work hard to improve their products and over time, star ratings gradually improve. So when the stars were re-graded in 2000 for all product types, it was necessary to leave some room for improvement. Hence under the current system, the most efficient products are generally only 3 or 4 stars (although there are some products that rate nearly 5 stars already for some appliance types). Because manufacturers have achieved substantial increases in the energy efficiency of refrigerators and air conditioners since 2000, the star rating algorithm for these two products was again re-graded in 2010 and the star rating system expanded to allow up to 10 stars for products with exceptional efficiency.

Detailed star rating equations

The star rating equations for most products are fairly complex. However, if you are interested in the mathematics of how it is done, the technical details are contained in the document “Equations for Appliance Star Ratings”. This document shows the star rating algorithms (equations) for refrigerators and freezers, dishwashers, clothes washers, clothes dryers and air conditioners. Equations for the old system (pre-2000), for the current star ratings (from 2000 for most products) as well as the new star rating equations for refrigerators, freezers and air conditioners from 2010 are provided for information. It also has information on star ratings for gas appliances as well as water star rating equations for dishwashers and clothes washers.

Power Input (also called Comparative Energy Consumption or CEC)

The energy consumption or power input of an air conditioner is measured under conditions specified in an Australian Standard. Because the heating and cooling requirement is affected by climate and this varies substantially across Australia and New Zealand, air conditioner use is not shown on the energy label. Instead, the cooling and/or heating Output and the Power Input is shown on the energy label at rated capacity (the units on the label are in kW which is the same as kWh/hour). To work out the likely annual energy use will require information on the climate and other factors such as occupancy (hours that cooling is required) and building shell performance (insulation, glazing, orientation etc). It is important to note that under normal usage, the air conditioner will spend a significant amount of time at less than its rated capacity – in terms of efficiency this is really only important for variable output models which can have higher efficiency under part load conditions.

Capacity Output

The measure of energy service for an air conditioner is the rated cooling and/or heating capacity of the air conditioner, usually specified in kilowatts (kW) (some product brochures use BTUs (British Thermal Units), although this is now unusual and some retailers use compressor “horsepower”, although this has no meaning in terms of the units capability). These rated values are as declared by the manufacturers and under the test conditions are defined in the Australian/New Zealand Standard (which is based on the relevant international standard). The heating capacity of a reverse cycle air conditioner is the heat that can be put into a room. Similarly, the cooling capacity is the heat that can be removed from a room. The cooling capacity is made up of the sensible component (usually the majority of the capacity) which relates to the actual temperature reduction (cooling) of the air, plus the latent component, which is a measure of the dehumidification effect of the indoor air. Latent cooling capacity is sometimes expressed as moisture removal capacity in litres or kg of water per hour (1 kg per hour of moisture removal is equal to 683 Watts latent capacity).

How can the Capacity Output be greater than the Power Input?

Refrigerative air conditioners (the only type covered by energy labelling in Australia and New Zealand – evaporative units are not included) use a technique called the vapour compression cycle to “move” energy in the form of heat from one space to another. This is generally a very efficient process and the amount of low grade heat that can be moved is typically 3 to 5 times (or more) the energy required to run the compressor system. This ratio is called the Energy Efficiency Ratio (EER), used for cooling, or Coefficient of Performance (COP), for heating, and it is used as the basis for determining the star rating of an air conditioner (see below). A refrigeration heat pump collects internal heat and moves it outside when in cooling mode, or collects ambient heat from outside and moves it inside when in heating mode.

The efficiency of the system depends on the components used (their design and how well these are matched – compressor, evaporator and condenser) and the temperature different between inside and outside (as the temperature different increases, the system becomes less efficient).

The system uses a refrigerant (which exists as a gas at low pressure and as a liquid under compression) which is compressed and liquefied, allowed to cool in a condenser, and then allowed to expand in a controlled way (through an expansion valve) to become a gas in an evaporator (the expansion is accompanied by a strong cooling effect). In this operation the condenser becomes warm and the evaporator to the condenser.

The principle is the same as used in a normal refrigerator which “moves” heat from the inside of refrigerator to the outside. In the case of an air conditioner when in cooling mode, the heat is removed from the room being cooled and pushed outside through the refrigeration system. Similarly, if the unit can operate in “reverse” (called heating mode or reverse cycle), the process runs backwards and the energy is collected from outside and moved inside to the room being heated. In most cases, air conditioners are more efficient when operating in heating mode as the energy used to compress the refrigerant can also contribute to the net heating output.

Performance

To be eligible for an energy label (and to comply with MEPS), an air conditioner must meet the maximum cooling test as defined in the Australian/New Zealand Standard – this ensures that the air conditioner is capable of operating under extreme conditioners. The air conditioner also has to have a tested capacity of not less than 95% of the rated value and a tested energy consumption of not more than 105% of the rated value.

Star Rating

The star rating for air conditioners is determined differently to other appliances. For air conditioners, the measure of energy efficiency is the Energy Efficiency Ratio (EER) for cooling and the coefficient of Performance (COP) for heating. The EER and COP are defined as the capacity output divided by the power input. The Star Rating Index is calculated on the measured values for energy and capacity during a rating test, rather than the nameplate or rated values.

The original star rating equations for air conditioners were developed in 1987. These were revised (re-graded) in 2000 and again in 2010 to take account of the substantial improvement in the energy efficiency of products over this period. Until 2010, all energy labels showed possible star ratings from a minimum of 1 star to a maximum of 6 stars. In 2010, the star rating system for refrigerators and air conditioners was expanded to show up to 10 stars for products that have exceptional energy efficiency. Products that achieve up to 6 stars continue to use a normal 6 star energy label.

You can view the samples of the Air conditioner energy label here.

The star rating for air conditioners is determined from the measured EER and COP. From 2010, for cooling, 1 star is equal to an EER of 2.75 with an extra star for an increase in EER of 0.5. For heating, 1 star is also equal to a COP of 2.75 with an extra star for an increase in COP of 0.5. Importantly, the 2010 star rating system is based on an annual efficiency calculation which includes any non-operational energy consumption such as standby and power consumption of crank case haters (where present).

Air Conditioner MEPS

Since October 2004 in Australia and 2006 in New Zealand, all single phase air conditioners have been required to meet Minimum Energy Performance Standards (MEPS), which specify minimum levels of energy efficiency for these products. All three phase air conditioners have been required to meet MEPS since October 2001 in Australia and 2006 in New Zealand. Revised MEPS levels were introduced in April 2010 in Australia and upgraded, most recently in October 2011. You can find details of the current and future MEPS levels for air conditioners on the web page that describes MEPS for air conditioners in detail.

Proposals for increased future MEPS are under consideration. Details are included in Regulatory Impact Statements (RIS).

The detailed star rating equations are contained in the document “Equations for Appliance Star Ratings”.