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How the Efficiency of Air Conditioners is Rated and Calculated

You may have heard the term “SEER” used to describe the efficiency of your air conditioner. But, what does it mean? There are a couple terms used to describe residential and small commercial building air conditioning efficiency. Two terms that are used are Energy Efficiency Ratio (EER) and Seasonal Energy Efficiency Ratio (SEER). Another term that is used is COP.

The term EER is used mostly for small room air conditioners (i.e. window units). The term SEER is used for central air conditioners.

Both of these terms are technically ratios that measure how efficiently a cooling system operates. The EER or SEER rating is written on the Energy Guide Label that is attached to a new air conditioner. Some air conditioner manufacturers participate in the “Energy Star” labeling program, which indicates that they have higher EER and SEER ratings.

First, we will discuss EER. EER is a measure of how efficiently a cooling system operates when the ambient (outdoor) air temperature is at a specific level (usually an ambient temperature of 95oF is used). As EER increases, the energy efficiency of the system increases. Higher EER = Higher Efficiency.
Mathematically, EER is expressed as:

EER = qc / p         (1)
qc = output cooling energy (BTU/hr)
p = input power consumption (Watt)

EER is usually used for room air conditioners ranging 5,000 BTU per hour to 15,000 BTU per hour. In order to determine EER, we need to know what the BTU is of the system. That leads us to another question. What is a BTU?

A BTU is a measurement of energy, and BTU/hr is used in the study of heat transfer. “Heat transfer is the study of thermal energy transport within a medium or among neighboring media by molecular interaction (conduction), fluid motion (convection), and electromagnetic waves (radiation), resulting from a spatial variation in temperature ” [3]. In the case of typical forced air conditioning, the type of heat transfer is convection.  A BTU, or British Thermal Unit, is the amount of energy needed to heat one pound of water by 1oF.

The “Ton” is frequently used to express air conditioning power, especially on older units. A “ton of refrigeration” is the heat energy absorbed when 1 Ton (2000 lbs.) of ice melts over 24 hours. Since air conditioning power is usually expressed in Tons, we need to convert Tons to BTU/hr, in order to calculate SEER. To do this, one needs to know what the latent heat of fusion (melting) of ice. Latent heat is the heat energy absorbed while a substance is changing phase (from a solid to a liquid, or from a liquid to a vapor), without a change in pressure or temperature. For ice, the latent heat is 144 BTU/lb. To get BTU/hr, multiply the latent heat by the weight of the ice (2000 lbs.), and then divide by 24 hours.

1 Ton = 2000 X 144 / 24 = 288,000 BTU / 24 hr = 12,000 BTU/hr

Selecting a Window Air Conditioner
So, which window air conditioner should you buy? In mild climates air conditioners with an EER of at least 9.0 should be selected. In hotter climates air conditioners with EER over 10.0 should be used.

Now, we will discuss SEER. SEER is Seasonal Energy Efficiency Ratio, and is a combination of the EER and the COP (coefficient of performance). SEER is similar to EER, but the wattage used is the actual wattage. So, more power is used when the unit starts up than when it has been running for a little while. The actual wattage includes the average number of starting and running cycles. The actual wattage used is calculated by running the unit in a fixed environment. This number is listed on the unit’s nameplate. SEER is mathematically expressed as:

SEER = Q c / P         (2)
Q c = Sum of cooling outputs at all test conditions (Btu/hr)
P = Sum of all watt-hour inputs at all test conditions (Watt-hours)

You should make sure that the SEER is at least 13 (as of the year 2006). 

An example of calculating SEER is as follows:
You purchased a 5/6 Ton air conditioning unit. This is 10,000 BTU/hr. The label on an air conditioner reads that the Watts Input is 800W.
SEER = 10,000 BTU/hr / 800W = 12.5.

COP is the Coefficient of Performance of a machine. This is the ratio of power output (Watts) to power input (Watts). COP can be used for any machine, not just an air conditioner or heater. COP can easily be calculated from SEER. To convert SEER to COP, convert the BTU energy and the electrical input energy to a common energy unit, the Joule. One BTU = 1055 J. 1 W-hr = 3600 W-s or 3600 J. Therefore:

            COP = SEER (BTU/W-hrs) X 1055J/BTU / 3600J/W-hr = SEER X 0.293           (3)

If SEER is 12.5, then COP = 12.5 X 0.293 = 3.66.

Because SEER is calculated in a fixed environment, the actual efficiency of the system may vary widely depending on where the air conditioning unit is installed and what the temperatures, ductwork layout, and other operating parameters are. The efficiency can also decrease if a unit is not well maintained (cleaned and recharged if necessary) on a regular basis. Therefore, it is very important to have a well-trained technician install and maintain one’s air conditioning system.


  3. Kaviany, M. 1999. Principles of Heat Transfer. Departments of Mechanical Engineering & Applied Mechanics, The University of Michigan, Ann Arbor, MI.
  5. Althouse, Turnquist, and Bracciano. Modern Refrigeration and Air Conditioning, 18th ed. Tinley Park, IL: The Goodheart-Willcox Company, Inc., 2004.


This content was written by Valerie Johnson and is copyrighted 2012.