Before purchasing any air conditioner, it’s important to figure out what size unit you need. If it's too small, it has to work harder to reach the target temperature, wasting energy. If it’s too big, it will cool the room before sucking out moisture, leaving you feeling clammy. Figuring out the correct size AC for your room involves some math, but it’s pretty easy. First, measure the length and width of the room you want to cool, then multiply those numbers to calculate the square footage.  In general, you want 20 to 25 British Thermal Units (BTUs) of cooling power for every square foot. So, multiply the square footage of your room by 20 to get the minimum size air conditioner that will work in your space. If your room is 500 square feet, for instance, you need an air conditioner with 10,000 BTUs. If you have high ceilings and large windows, you might need more cooling power.  Energy Star recommends increasing capacity by 10% if the room is very sunny, reducing it by 10% if it’s heavily shaded, adding 600 BTUs if more than two people regularly occupy the room, and increasing capacity by 4,000 BTUs if the unit is in a kitchen.