Our comfort levels depend as much on heat levels as on moisture levels, and air conditioning is as much a matter of dehumidification as it is of refrigeration. And that’s why you should pay attention to both the energy efficiency rating of the AC you want to buy (given by its EER or its SEER) and its SHF (Sensible Humidity factor), a coefficient that measures the ability of the air conditioner to remove humidity.
The Energy Rating for room AC
Prefer room air conditioners (and one air-handler ductless mini-splits) with a high EER (Energy Efficiency Ratio). You will get significant energy savings, and it will not cost you significantly more.
Energy Star certified products have at least a 10.7 EER, which means 10% more efficiency than a 9.7 EER unit, or 20% more efficiency than a 8.7 unit...
In North America, new air conditioners systems are required to carry a yellow EnergyGuide label, which lists their energy consumption and their estimated annual running costs. See also, for European countries: UE eco-labeling.
Central Air Conditioners SEER
Central air conditioners and ductless mini-splits are rated by their SEER (Seasonal Energy Efficiency Ratio). Like the EER rating of room air conditioners, the SEER measures the amount of heat that the AC is able to remove. So, the higher the SEER the better…
Prefer units with a SEER above 15, and a qualified Energy Star model. Also prefer two/multi-stage AC air conditioners with a quiet condenser, that is, with a sound level of 75 decibels or less.
The humidity coefficient and the energy Savings of AC
Also very important – both for central air conditioners and room and portable units – is the effectiveness of the air conditioner at removing humidity. As mentioned earlier, air conditioning is as much a matter of heat removal as it is of humidity removal; human comfort depends both on heat and moisture levels.
So, when buying, do not forget the SHF (Sensible Humidity Factor), a coefficient that measures the ability of the air conditioner to dehumidify the air efficiently (the SHF is expressed by a decimal number between 0,5 and 1).
Just do not forget that the SEER/EER and the SHF coefficients oppose each other: the higher the capacity of the air conditioner to remove humidity, the lower its capacity to cool; and you should find a balance between the two.
That is: in dry climates, where moisture removal is not important, look for an air conditioner with a high SHF, i.e., with a low moisture removal capability. That's important for low-energy consumption.
But in humid climates you should look for a balance of both reasonable SEER/ER and SHF.... Look for models with a SHF coefficient between, say, 0,67 and 0,77. These models will be good at reducing moisture (and mold, mildew and fungi) without being significantly affected in their efficiency.
The energy efficiency of portable air conditioners, like that of room air conditioners, is rated by their EER coefficient.
Unfortunately there isn't any organization to certify these units and their EER; the information provided by manufacturers is of doubtful credibility and accuracy. Non-vented units are very poor choices, and other units may have intrinsic flaws and high-energy consumption. See: Portable AC.