Cooling Capacity is the cooling power of a central air conditioner. It is most commonly measured as the BTUs per hour of heat that the air conditioner can remove from the air. (You may also see cooling capacity rated in tonnage.)
The only accurate way to determine the cooling capacity that you will need in a residential or non-residential air conditioner is to have a heating and cooling contractor perform a load calculation. This calculation will take into account many factors about the building that you want to cool: its size, number and direction of windows, amount of shade, amount of insulation, etc.
An air conditioner whose capacity is too small will not adequately cool your residence or building. If the capacity is too large, it will cycle on and off too often, decreasing efficiency and increasing your electric bill. In addition, it will not maintain proper humidity levels, resulting in a less comfortable environment.