When considering what kind of air conditioner to purchase for your home or for a single room of your house, you will want to pay close attention to the Energy Efficiency Rating (or EER) of the different units. What is this EER that is so frequently spoken of for air conditioners and refrigerators, you ask? Such a good question should be addressed so that you can make the best possible decision for your next air conditioner.

As stated earlier, EER stands for Energy Efficiency Rating. Standardized by the Air-Conditioning and Refrigeration Institute, this rating reports central air conditioning efficiency at 95 degrees Fahrenheit outdoors and 80 degrees F indoors. EER is a measure of the amount of electricity needed by an air conditioner to cool a room or house to a desired level of BTU (British Thermal Units). Moreover, EER applies primarily to commercial air conditioning systems and measures the “steady-state” efficiency of air conditioners. This means that EER tells the efficiency of the air conditioner once it has already been running for a period of time (due to the fact that more energy is required to get the air conditioner started). The rating system consists a ratio of desired BTU level to wattage usage to cool to that level. For example, an air conditioner that can cool 10,000 BTU using 1000 watts would have an EER of 10. Generally speaking, a EER of 10 is the minimum of new units.

In relation to EER, another rating used by the Air-Conditioning and Refrigeration Institute is the Seasonal Energy Efficiency Rating (or SEER). SEER is only applicable to residential or consumer cooling units and is often considered to be a more correct and reliable indicator of the overall energy efficiency than the standard EER. Currently, air conditioners must have an SEER of at least 13 to be sold in the U.S. Some air conditioning units have an SEER of up to 21. Though SEER is often criticized for not taking into account the air conditioner's efficiency while under partial load, even though there is no widely available rating of residential air conditioner efficiency under partial load.

One rating system that does address the issue of partial load efficiency is the Integrated Part Load Value (or IPLV). Also developed by the Air-Conditioning and Refrigeration Institute, IPLV measures partial load efficiency only in commercial cooling units. Contrary to the EER, which measures commercial air conditioners at their “steady-rate”, IPLV measures commercial air conditioner efficiency in a variety of conditions – namely, at 25%, 50%, 75%, and 100% capacity and at different temperatures.

Another rating to take into consideration is the “fleet efficiency” or “air conditioner rating”. The fleet efficiency measures the aggregate efficiency of all cooling systems built by a particular brand. The fleet efficiency tells nothing about the air conditioner quality or the durability of a certain brand's air conditioners, but it will give you an idea of how much it will cost you to run an air conditioner of different air conditioners. Because it only measures efficiency, it doesn't necessarily indicate that a particular model from one brand of air conditioner outperforms any other model of air conditioner built by a different company.

When purchasing an air conditioner, whether it is a commercial, residential, or portable air conditioner, be sure to look at the yellow tags that are required by federal law to be displayed by the manufacturer. These tags show the EER of the air conditioner as well as an estimated annual operating cost for the given unit. Probably the most convenient element of the yellow EnergyGuide tag is a sliding scale which helps you compare different units easily as its EER is plainly displayed on a scale from 10 to 16.9. Consult with a qualified air conditioner technician to determine the correct capacity that you will need to cool the desired area. Also, look at many different models and brands of air conditioners to make sure you find the best possible unit for your needs.