If you’re installing any kind of air conditioner, be sure to get the right size. A unit that’s too small won’t cut the heat, but an oversized system wastes energy. Match room size with cooling capacity, measured in British thermal units, or Btu: the quantity of heat required to raise the temperature of 1 pound of water by 1 degree Fahrenheit at a specified temperature. Follow these steps to determine what size unit will maximize your cooling.

1. Calculate the square footage of the area to be cooled.

2. Determine the best approximate cooling capacity using this chart (capacity will differ depending on ceiling height).

Square Feet           Btu
550 to 700            14,000
700 to 1,000         18,000
1,000 to 1,200      21,000
1,200 to 1,400      23,000
1,400 to 1,500      24,000
1,500 to 2,000      30,000
2,000 to 2,500      34,000

3. Adjust measurements in these situations:

• Reduce capacity by 10 percent for shaded rooms.
• Increase capacity by 10 percent for sunny rooms.
• If more than two people regularly occupy a room, add 600 Btu for each additional person.
• If a unit is used in a kitchen, increase capacity by 4,000 Btu.

And consider where you install a unit: If you’re mounting an air conditioner near the corner of a room, look for a unit that can send the airflow in the right direction.

Source: www.energystar.gov