The fact is that a room air conditioner that’s too large for the area will perform less efficiently then a properly sized unit. And a unit that is too small for the room will not have enough BTU’s (British Thermal Units) to cool the room at a comfortable cooling level. The cooking capacity of the unit is measured in BTU’s (British Thermal Units), the more BTU’s it has , the more space it will cool down. If the unit doesn’t have enough BTU’s, the room never will become cool and comfortable. So before you buy that air conditioner you first need to figure out what size you will need for the room your looking to cool off.