Air Conditioning units are all based on the same concept: removing heat and humidity from the air in your home and discharging the heat and water outside. A term used to describe the efficiency of accomplishing this work is named a SEER. SEER (Seasonal Energy Efficiency Ratio) addresses this issue, as it can be very confusing to a consumer. Automobiles provide the best analogy for this, the higher the MPG the further you can go, and would you prefer 16 MPG or 28 MPG.
The air conditioning SEER is the same concept. The higher the SEER the better the mileage, or the less you pay in fuel cost.
The unit itself must be designed with more intricate parts in order to operate efficiently enough to raise the SEER rating which in turn means the unit initially will cost more, but will save money in the long run. This is research the consumer needs to consider before purchase in order to consider the payback. To learn more, you can visit The Air Conditioning, Heating and Refrigeration Institute AHRI.