The star rating is about energy efficiency – that is how efficient a model is relative to other models of the same size.
More stars means more efficient – when compared to other models of the same size.
Most products are given between 1 and 6 stars. However technology keeps getting better… as does energy efficiency! This is why nowadays you’ll see some super-efficient models in shops and online with an extra row for stars, as they can have up to 10.
Tip: Always choose which size (or capacity) model you need first, then use the star rating to compare them.

Energy ratings for gas hot water systems
Gas energy ratings are based on a set standard of gas use to perform a certain task over the course of a year. For gas hot water, it is 37.7 megajoules (MJ) per day of hot water delivered, which equates to 13761 MJ per year. The difference between this standard and the total energy used by the water heater (covering things from burner efficiency to heat lost through the tank insulation) gives the heater its comparative energy consumption rating. This rating is an indication of how well the hot water heater performs against the national standard.
It is quickly becoming mandatory for 5 star and above hot water systems to be installed in new homes, so if you are seeking to build, make sure the system you choose complies with your state’s requirements. For South Australia you can check HERE

Energy ratings for electric hot water systems
Electric energy ratings are more familiar, as they appear on many other common appliances such as refrigerators and air conditioners. These ratings are called Minimum Energy Performance Standards, or MEPS. The equation used to determine the energy efficiency ratings of electric hot water systems is similar to the one for gas in that there is a set standard that all electric water heaters are measured and rated against. However, electrical systems are broken down into ranges of allowable heat loss for a given amount of hot water delivery. Heat exchange systems (like heat pumps and geothermal systems) conform to a slightly different standard than standard electric storage systems.

for more detailed information visit the ENERGY RATING website

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