Your scepticism and reasoning are spot on. There is clearly nothing 'in it' at all, except for the money that ends up in their pocket. 
However, there IS a simple, zero cost way of reducing your aircon power consumption, simply turn the setpoint up a couple of degrees.
I recently bought a split-system inverter aircon, and spent a bit of time studying the efficiency of competing brands and models, which is characterised by the EER rating. The very best inverter units these days have an EER (Energy Efficiency Ratio) of up to around 4.8, meaning that they can produce 4.8kW of cooling for every kW of electrical input. For reverse cycle heating, the equivalent ratio is known as the COP, which usually has a similar value.
What I found was that the smaller capacity units almost always had the highest EER ratings, which initially puzzled me because, in general, machines become more efficient as they are scaled up. The explanation lies in the fact that to save design and production cost, the basic components are only made in a small number of physical sizes. For example, although a manufacturer may produce a range of split-system units from 2kW cooling through to 8kW cooling, the indoor and outdoor units are physically in typically only 2 sizes, with the different power ratings achieved by varying the power of the compressor, and the fans. So, what you find is that the external dimensions of the indoor and outdoor units will (for example) be identical for all units from 2kW up to 5kW, and the effeciency (EER) is best for the 2kW unit, and gets progressively worse up to the 5kW unit. Then, a larger indoor and outdoor case size is used for the 6,7 and 8kW units. As a result, the EER is better for the 6kW unit, and then gets progressively worse again for the 7 and 8kW units.
The effiency (EER) of an aircon depends largely on the physical size of the finned heat exchangers in the indoor and outdoor units, relative to the cooling power. So, in a 2kW unit, you have a small, low power compressor, with relatively large heat exhangers and fans, so the EER is very high. However, rather than scale up the size of the enclosures and heat exchangers for the higher capacity models, the manufacturer saves money by keeping the same enclosure (and heat exchanger)size right up to 5kW capacity, so EER drops accordingly. As an engineering purist, I find that obscene. Personally I would pay extra money for a (physically larger) 5kW unit with an EER=4.8, but such a unit is not available. Consumers are partly to blame here as well, for preferring that everything be as small and compact (and therefore inefficient!!) as possible.
To continue the story, the indoor and outdoor units jump to the next physical size for the 6kW unit, so we observe an upward jump in the EER, but not up to 4.8 again, as to do that would require indoor and outdoor units that would be physically larger and more expensive than consumers would tolerate. Then the EER drops again to painfully low values as the case size stays the same, and a larger compressors are slotted in up to 8kW capacity.
If anyone is seriously interested in this stuff, I'll dig out the exact numbers for the Mistubish Electric range, and you will be in no doubt that the above observations are true.
An interesting question arises as to whether the EER increases when any given unit is operating below rated cooling power, which would happen frequently with an inverter unit that is correctly sized, and therefore not permanently operating flat out.
The 'star ratings' for aircons reflects what I have said, with the smaller units scoring the highest number of stars, and I hope that (like me) you found it interesting to know why that is so. However, I suspect the star ratings may be misleading, for it may well be that a 6kW unit (with large case size & heat exchangers) operating at half capacity could be more efficent that a 3kW unit operating at full capacity.
Posted Tuesday 17 Jan 2012 @ 9:10:11 am from IP
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