The star rating labels on modern air conditioners will have a star rating for cooling and another for heating. The basis for these ratings is the energy consumed to perform each process. In cooling the EER or 'energy efficiency ratio' is used. This is simply the output divided by the input. For example if your air conditioner has a 6KW (6000 watts) cooling capacity and an input of 1.5KW (1500 watts) then the EER is 6 divided by 1.5 equals 4. In heating, the COP or 'co-efficient of performance' is used. This is more difficult to explain however an attempt is made in the article "How does an air conditioner work? in this blog.
Now if maths calculation alone were used the star rating would be a reliable reference, but a 'lab testing criteria' is included which can significantly alter the outcome. This is because of the special features included in some products greatly alter the efficiency measurements. So lets look at a few of these features and you'll see what I mean.
An inverter compressor can produce 110% of its stated capacity. In other words a 5KW product can actually produce 5.5KW. Some manufacturers have taken to limiting the range to the stated capacity and thus reducing the effective size of the air conditioner. The reason for this is to remove the extra power required to achieve this extra 10% and appear to consume less energy for the same rated size. Of course you're actually paying for a smaller product with an often greatly inflated price.
Special features that distort Star Ratings
My favourite is the intelligent eye. One of the functions of this is to detect when there has been no heat source in the room for twenty minutes and switch to economy mode. Some models warn that this sensor can mistake pets for humans, can in fact mistake any heat source for a human, and can fail to recognize an actual human if they haven't moved for a while. A little advice here is that if you have one of these sensors on your bedroom unit be sure to wake up every twenty minutes and wave, just to let the air conditioner know you are still there.
A variant of the intelligent eye is the sunlight sensor. This claims to detect the intensity of sunlight, from this calculate the temperature and adjust accordingly. If it doesn't detect sunlight it assumes it is dark or cloudy. Got that?
Economy mode has no universal meaning other than 'more economical than before'. One variation alters the set-point upwards 2 degrees Celsius (in cooling mode) thus consuming less energy. Another runs the unit at a reduced cooling capacity regardless of the room temperature. In other words you can set the temperature to suit you, or you can let the machine set the temperature to save power.
When measuring star ratings it is assumed that these special features are used much or all of the time. The point I'm trying to make here is that the impact of special features on star ratings is significant. Despite my own obvious skepticism, the real question is:
Do more stars mean more savings?
In a nut shell...it depends. A recent CSRIO study into building energy efficiency ratings found that 31% of all energy consumed by residential air conditioners in Queensland was by letting the timer function turn the unit on and off automatically. A recent trend is to incorporate wireless connectivity to allow remote control via smartphones. This can only increase this wasted cooling energy. A correctly sized air conditioner does not need to turn on before you get home. An obvious impact on efficiency is whether the special features that you are paying for are things you will actually use or need. In other words... buy the air conditioner with the features you will actually use and then use them.
Five Points to Consider About Star Ratings
- Air conditioner performance is based on EER and COP figures. These are the numbers you should be looking for when considering efficiency ratings.
- Star ratings are influenced by special features.
- Consider whether you will use the special features. If not, the number of stars is meaningless.
- Stars cost money. Beware inexpensive units with many features and high star ratings.
- Star ratings are designed to assist you choose a unit. The real energy savings are through intelligent use.