A few basic concepts to understand when you are shopping for an air conditioner are BTU, EER, and SEER.

BTU stands for British Thermal Units. Air conditioners are given BTU ratings to show their cooling capacity. In technical terms, one BTU is the amount of heat needed to bring the temperature of one pound of water up by one degree Fahrenheit. You may have also heard of air conditioner capacity referred to by the ton. In the parlance of HVAC (Heating Ventilation and Air Conditioning), one ton equals 12,000 BTU. You can use BTU ratings to determine what size air conditioner you need to cool your space. Generally speaking, you need 20-30 BTU per square foot that you need cooled. However, if the area to be cooled has factors that increase or decrease the load on the air conditioner, you will need to alter your estimate of your BTU requirements. Here is a chart that estimates BTU requirements per square foot:

Area To Be Cooled (ft2)
Estimated Capacity (BTU/Hour)

Here are a few factors that might increase or decrease the BTU capacity that you require:

  • Reduce your required capacity by 10% if your room is heavily shaded
  • Increase your capacity by 10% if your room is very sunny
  • Add 600 BTU to your capacity each additional person (beyond two people) that regularly occupies the room
  • Add 4,000 BTU if the room is a kitchen
If you would like an interactive estimate of the BTU capacity you require, please check

EER stands for the Energy Efficiency Rating. Essentially, the EER lets you know how much cooling capacity from a given amount of electricity. The higher the EER, the more energy efficient an air conditioner is. If you are comparing two air conditioners with similar BTU capacity, you might check out their EER to help you make a decision. An air conditioner's EER is found by dividing its BTU capacity by its running wattage.

Wattage = EER

To put it another way, the higher the EER, the less energy the air conditioner consumes in order to produce cooling. However, units with higher EER generally cost more up-front than less efficient units. In order to decide between a more efficient and a less efficient model, you might perform a little calculation, as shown in the box below:

How to calculate the value of EER ratings
To compare the value of two different air conditioners with equal BTU capacities, you need to know a few things:
  • The estimated number of hours per year that you would use the air conditioner
  • The estimated cost per kilowatt hour (kWh) for energy
  • The wattage of the two different units
  • The cost differential between the two different units
Now you can use this formula:
Hourly usage per year times Wattage difference between the two units divided by 1000 equals your savings in kilowatts (kWh) per year.
Now, multiple this times Your cost per kWh (you can find this on your electric bill) and it equals
The amount of money saved per year by using the more efficient (higher EER) air conditioner.
Now, to find the value of the air conditioner with the higher EER, simply take the cost differential divided by the amount of money saved per year and this will equal the number of years to recoup the cost differential between the two units.
If you would like to compare two air conditioners and calculate your cost savings, you can use one of the following forms:
Microsoft Excel worksheet
Adobe Acrobat worksheet
You will need the appropriate program to open the above files. Adobe Acrobat is a free download, if you do not already have it.
Also, the Adobe Acrobat document is best to print, but the Microsoft Excel document will do the calculations for you.

Generally, an EER of 10 or above is considered good, and EnergyStar air conditions have are rated with a 10.7 EER or higher.

More information on EER: How Air Conditioners Work- BTU and EER
ACE3: Consumer Guide to Home Energy Savings - Air Conditioning

SEER stands for Seasonal Energy Efficiency Rating. It is similar to the EER, but SEER is usually used to rate central air conditioning units, while EER is generally used for window units and other, smaller air conditioners. Again, SEER is a measure of how efficiently an air conditioner turns electricity into cooling. The higher the SEER, the more efficient the air conditioner. SEER is figured out by dividing the Annual BTU Cooling divided by Total Watt Hours equals SEER.

Annual BTU Cooling
   Total Watt Hours    = SEER

As with smaller air conditioners, with other features being equal, the up-front cost of an air conditioner with a higher SEER will likely cost more than a system with a lower SEER, but the higher SEER air conditioner will use less money over its life, and might be a more economical choice in the long term. Another factor to consider is that an older central air conditioner might have a substantially lower SEER than a new central air conditioning unit, so it might make financial sense to replace an older central air conditioner with a newer, more efficient model.
Generally, a SEER of 12 or above is considered very efficient, although some central air conditioners claim SEER approaching 20!