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, 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. But 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.

Comparative energy consumption CEC

The energy consumption of a clothes washer is measured under conditions specified in an Australian Standard. Over a year, it is assumed that the clothes washer is used 7 times per week at rated capacity on a warm wash (warm CEC in red). A value for a cold wash energy of 7 times per week is also shown on the label (cold CEC in blue). The clothes washer is labelled on the “normal” or “regular” program (program specified for a normally soiled cotton load). The energy consumption of a clothes washer includes both electrical energy for motors and pumps and the energy embodied in any imported hot water or electrical energy used to heat the water internally. The majority of energy for a clothes washer is to heat water on a warm wash.

Capacity

The measure of energy service for a clothes washer is rated load capacity. This is the value declared by the manufacturer and defines the test load used in the Australian Standard.

Performance

To be eligible for an energy label, a clothes washer must be able to meet a minimum level of wash performance, a minimum level of spinning performance and a must not exceed the “wear and tear” limits that are defined in the Australian Standard.

Base Energy Consumption (BEC) and Star Rating

The Base Energy Consumption defines the “1 star” line for particular products. An additional star is awarded when the CEC of the model is reduced by a defined percentage from the BEC. The energy reduction per star is 27% for clothes washers. For example, a model that had a CEC that was 0.73 of the BEC or less would achieve 2 stars. Similar, a CEC of 0.533 (0.73 x 0.73) of the BEC or less would achieve 3 stars and so on.

For clothes washers, front and top loading models are rated on the same basis. The warm wash energy consumption and a component of residual moisture (spin performance) is used to define the star rating in comparison with the BEC. Therefore a model that has a good spin performance may get a marginally higher star rating than a model of the same capacity and CEC with a poor spin performance.

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