An air conditioner suitable for a 12 by 15 room has a minimum cooling capacity of 4500 BTU. Estimate this by multiplying the area of the room by a constant: 25 BTU. Therefore, 12 x 15 = 180 square feet. When 180 is multiplied by 25, the result is 4500.
If a smaller air conditioning unit is used, it is going to run often and use a great deal of electricity. Even so, it is not likely to cool the room sufficiently. However, if an air conditioner is too large, it only does half of its job. As the appliance makes a room cooler, it also removes moisture from the air. A large air conditioner in a small room does not need to run very long, so too much moisture remains. This makes the cooler air feel humid or clammy.