To figure out the size of air conditioner that is needed for a space, the square footage needs to be established so that the cooling capacity can be determined. It is important that the air conditioner is not too small or too large for an area as either of these can waste energy.
The cooling capacity of an air conditioner is measured in BTUs, or British thermal units. The amount of BTUs an air conditioner needs to have per hour to cool a space greatly depends on the square footage of the space in question. Once the amount of BTUs is established, the air conditioner can be picked out based on its BTU output. The website for Energy Star has a chart that shows the BTUs required for spaces with 100 to 2,500 square feet areas. The website for Good Housekeeping has a BTU calculator where information about the space can be entered.
There are other factors that need to be considered when picking out an air conditioner as they affect the amount of BTUs needed for a space. These include the amount of insulation, hours of sunlight the space receives, location of the space and the space's number of regular occupants.