When you are trying to determine what size air conditioner to get, the first thing you need to know is how large the space you want to keep cool is. Air conditioners come in a wide variety of sizes and each is appropriate for a particular type of space. So once you know how big the space is, you will be able to compare that to the capacities of the various air conditioners out there to find the right one for your needs.
The cooling power of air conditioners is actually measured in British Thermal Units, or BTUs. Each BTU rating corresponds to a particular square footage which that air conditioner should be able to cool properly and efficiently. For instance, a 10,000 BTU air conditioner can cool a room that measures between 400 and 450 square feet, and so forth. The higher the BTU, the larger the area the air conditioner can acceptably cover.
It is pretty easy to see why you would not want to pick out an air conditioner that is too small for the space you need to cool. The system simply would not be able to keep the space cool enough and it would use a lot of energy trying.
However, that does not mean that the bigger the air conditioner is the better. In fact, you will do just as poorly to purchase an air conditioner that is too big for the space you want to cool than one that is too small. This is because air conditioners do more than cool the room. They also remove humidity as they cool the space.
An air conditioner that is too large will cool the room quickly, causing it to switch on and off frequently. Not only is this annoying because of the noise that the air conditioner makes, but it also increases the wear and tear that the unit must endure. An air conditioner that is too big will also not be able to remove humidity as efficiently because the room is cooled so quickly. Plus, it will use much more energy than necessary to do a sub-standard job.
With these types of calculations, it is relatively easy to pick out a single room air conditioner. However, if you are trying to purchase a whole home air conditioning system, you will probably need a professional to help you figure out exactly how powerful you need the system to be to keep your home comfortable and cool all summer long.