Window air conditioners, also known as portable air conditioners, are intended to cool one room or area. They usually come in two types: models that can be installed in most double-hung windows, and models that fit in narrow, vertical windows, which usually require removing the window panel. The second type of room air conditioner is a wall-mountable model, which is also known as 'through-the-wall' and is encased in a sleeve that is installed in a hole through your walls to the outside.
The most important consideration in choosing a window or room air conditioner is size, with bigger not being necessarily better. Simply put, an improperly sized air conditioner, regardless of its EER rating, will not operate properly or efficiently and will therefore break down sooner, cost more to run and have negative impacts on the comfort level and air quality you had in the first place.
Size refers to cooling capacity in British Thermal Units, or BTUs, and the required size is determined by calculating the square footage of the room or area to be cooled, ceiling height, number and placement of windows, humidity of your climate, and even the number of people usually occupying the space.
Room or window air conditioners on the market today are far more efficient than ten years ago, and are required to have a minimum EER (energy efficiency ratio) of 9.7% for units below 8000 BTU, and 9.8 for those with a BTU of 8000 to 13,999. The EER is the BTU per hour over the wattage, and the higher the EER, the more efficient the unit.
Many manufacturers are producing air conditioners that surpass the federal standards, and an air conditioner with the Energy Star logo will be at least 10% higher than that required by government. Buying a higher efficiency air conditioner will cost more initially, but you will likely offset that cost over time with lower energy bills, and the environment will benefit too.