How To Estimate The Right Size For An Air Conditioning Unit

Window air conditioning units are priced based on the amount of cooling they provide. Buying the right size will give you the cooling capacity you need without overpaying. Follow the directions here to find the best match.

Why Size Matters

If your air conditioning unit is too small, it will run constantly and still not cool the room properly. But one that is too large isn't the right answer either. An air conditioner that is too large will run intermittently and draw more electricity from the constant on/off cycle. It will also develop "cold spots" near the unit and "hot spots" elsewhere. This happens when an overlarge air conditioner drops the temperature quickly where it's located and shuts off before cooling farther out.

Estimating the base size

Air conditioner capacity is rated by BTUs (British Thermal Units). A base size is first calculated using the square footage of the room where the unit will be installed. The scale starts at 100 square feet for a 5,000 BTU air conditioner. A thousand BTUs is added for every additional 50 square feet, up to a room size of 550 square feet (12,000 BTU). For even larger rooms, use this scale:

550 up to 700                   14,000

700 up to 1,000                18,000

1,000 up to 1,200              21,000

1,200 up to 1,400              23,000

1,400 up to 1,500              24,000

1,500 up to 2,000              30,000

2,000 up to 2,500              34,000

Additions to the base size

A window air conditioner relies on heat exchange to the outside of the building. If it is located on a south wall, where it will be exposed to more sunlight and higher outside temperatures, add 10% to the number of BTUs. Also add 10% to the base size if the room is sunny (consider using shades or blinds instead). People and large pets add moisture and heat to a room, which makes the air conditioner work harder. If the room is regularly occupied by more than one person or you have large pets, add 600 BTUs for each additional occupant. For rooms where heat is generated by appliances like computer equipment, add 600 BTUs. For a kitchen where a stove (not microwave) is in use, or where a clothes dryer vents into a room, add 4,000 BTUs.

Subtractions to the base size

If a room is on the north side of the house or is constantly shaded, deduct 10% from the base size. If you live in an area of the country where the summer humidity is very low (Arizona for example) deduct 600 BTUs. If the room will not be occupied during the hottest part of the day, and you will not be running the air conditioner then, you can also deduct 10%.

Using the final number

The correct number of BTUs you calculate is unlikely to match the air conditioners available. Round your number up to the next available size. For example, if you calculate you need a 19,000 BTU air conditioner (a size unavailable), you will buy a 21,000 BTU unit instead. For more information on finding an air conditioner unit suited to your home, visit HomeSmart Xcel Energy.