Air conditioners come in many different sizes, whether they are window air conditioners or central air conditioners. These units should be sized to cool the area of your home you’re trying to cool with the most efficiency. So how do you do that? Well, by measuring the area of the area to be cooled, you’ll have a pretty good idea what size air conditioner is for you.
To calculate this, simply multiply the length times the width of the room or area to be cooled. Then, as a practical number, multiply that total times 25 BTU. This allows ample cooling, whether it is a rainy, moist day or a hot, sunny, humid day. Let’s say the room is 12 feet wide by 15 feet long. That means 12×15=180 square feet. Take the 180 sq. ft. times 25 BTU per square foot and you get the minimum BTU air conditioner you should buy. That means 180×25=4500 BTU cooling capacity is needed.
You may ask if a smaller air conditioner will work or a larger one may be better? Here’s a thought about those questions. Smaller BTU units will continually run, just trying to keep up. This will increase your electric bill and it is unlikely the unit will be able to cool the area effectively.
To read more click here: http://electrical.about.com/od/heatingairconditioning/qt/airconditionersizing.htm