In an air conditioner, air is cooled when it passes over refrigerant coils, which have fins similar to an automobile radiator. The compressor sends cooled refrigerant through the coils, which draws heat from the air as it is forced over the coils. By using advanced heat transfer technologies, more heat from the air is transferred into the coils than in conventional models, saving energy required to compress the refrigerant. An oversized air conditioner is actually less effective than a correctly sized unit, and wastes energy at the same time.
Air conditioners remove heat AND humidity from the air.
If the unit is too large, it will cool the room quickly, but only remove a portion of the humidity. This leaves the room with a damp, clammy feeling. A properly sized unit will remove humidity effectively as it cools. Running a smaller unit for a longer time will use less energy to completely condition a room than running a larger unit for a shorter time. If the room is heavily shaded, reduce capacity by 10 percent.