AC Capacity: Sizing Your Air Conditioner to Save Money

Air conditioners on apartment building at sundown
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The more powerful your air conditioner, the better--right? Actually, no.

Not only is buying a higher capacity air conditioner more expensive upfront, it can unnecessarily run up your energy bills during the summer.

Even worse, it can leave your house feeling clammy. A too-small AC can be a problem too; it works too hard to cool the space and you still end up with high electrical bills. An air conditioner that is, as Goldilocks would say, "just the right size" not only cools the space, but it removes humidity at the same time.

Learn how to size your air conditioner for your entire house or a single room.

Doing the Math

Air conditioners' cooling capacity is measured in British Thermal Units, or BTUs. You may also hear AC size referred to in "tons," which is the equivalent of 12,000 BTU's. While it's not necessary to understand the science behind it, you should have a good grasp of capacity in relationship to the square footage you need to cool. Simple math skills will suffice.

First, find out the area of your room by multiplying the length by the width. For oddly-shaped rooms, reach back to basic principles you learned in geometry class. Once you have determined the entire square footage of your home or of the room you plan to cool, multiply it by 25 BTUs. The resulting number is the BTU capacity required. For example:

A 15 foot wide by 20 foot long room contains 300 square feet. Multiply this by 25 BTU, and you'll discover the room requires a 6,000 BTU capacity unit.



Keep in mind that this is a very rough estimate. It's pretty adequate for sizing a room AC, but other factors can also affect the size of AC you need for a particular space. Keep these factors in mind:

  • For a heavily shaded room, reduce the BTU capacity by 10%; if it is sunny, increase it by 10%.  Keep in mind that deciduous trees lose their leaves in winter, gain leaves in summer.
  • If more than two people occupy the room regularly, add an additional 600 BTU per each extra person.  An example would be a family room that gets heavy use by a family.
  • Kitchens produce a lot of heat, so add 4,000 BTU to your total.

AC Capacity's Other Determining Factors

When you purchase a new central AC, you will most likely consult an HVAC specialist or a large home improvement store that handles installations. Professionals typically use the scientifically accurate "Manual J" method to determine the right size. Still, it's a good idea to be familiar with the factors involved.

The size of your home is obviously one of the largest factors in determining the right AC size, but it's not the only one.  In fact, equally important are other factors that often get overlooked:

  • You must also consider your home's orientation relative to the sun.
  • How well it is shaded.  As mentioned earlier, you also need to take into consideration the type of trees.  For example, if the home is surrounded by evergreen trees, it will perpetually be shaded.
  • The number and size of windows as well as which directions they face.
  • How well your walls and air ducts are insulated.  If not, do you plan to insulate at some point in the near future?
  • Consider any major changes you plan to make to the house, such as:  building an addition, adding skylights or large windows, or adding another room.  All of these will significantly add to your ac needs.

Play around with an online calculator to get a general idea of how all these factors come together:

Summary

Determining AC capacity is a critical move you need to make before purchasing and installing the unit.  Once the unit is installed, it's a permanent fixture--so get it right the first time.

The electric bills you receive during the summer months are probably the highest of the year, thanks to your air conditioner. If yours is on its last leg or you've just moved into a new apartment and need a window unit, learn how to calculate the proper size. A properly sized AC rewards you with energy savings all summer long.