What is the Difference Between EER and SEER?


If you are looking for a new air conditioner for your Central Florida home, you’ve probably seen SEER numbers for different air conditioning or heat pump makes and models. Here’s what they mean:

SEER is a measure of an air conditioner’s energy efficiency. Yet there is also another common rating called EER. Although they both measure efficiency, they are also different and understanding them can help you make a more informed decision.


What is EER?

EER stands for Energy Efficiency Ratio. It was one of the first attempts to standardize how the energy efficiency of an air conditioner is calculated.

Typically, the condition for calculating EER is an outdoor temperature of 95°F and inside temperature of 80°F with 50% humidity.

How is SEER different?

The difference between EER and SEER is the “S”, which stands for seasonal.

Rather than measuring the energy efficiency of an air conditioner at one operating temperature, SEER is the calculation of how energy efficient the air conditioner is during the cooling season at varying temperatures.

Said another way, SEER takes into account how the EER is affected by different temperatures.

Where EER is calculated using a steady outside temperature of 95°F, SEER is calculated using a range of outside temperatures ranging from 65°F to 104°F.  The SEER uses three tests: one with humid indoor conditions, the second with dry indoor conditions and the third with dry conditions cycling the air conditioner on for six minutes and off for 24 minutes, all three using 80 degrees Fahrenheit inside and 82 degrees Fahrenheit outside.

 Which rating should I pay attention to?

The U.S. government only mandates that air conditioners display SEER. However, many still show EER, as well. Both ratings can be helpful in different situations.

  • EER is the efficiency you can expect from the air conditioner at peak cooling time (in the midst of the summer) because it is measured at only one, higher temperature.

  • SEER is an average. It takes into account the highs and lows of a typical home’s cooling pattern, but not necessarily a Southern home in hot, humid climates.

For areas of the country where it is 95°F or higher most of the time the air conditioner is on, the EER is usually more accurate. For more moderate climates, the SEER is a better measurement.


The SEER gives a seasonal average idea of the efficiency of your air conditioning unit, and even in Florida we have many hours during our air conditioning season that the temperatures do drop to 82 degrees (think nights and rainy days).

The important part

The most important part is to compare apples to apples. So make sure you are comparing one air conditioner’s EER to another’s EER (or SEER to SEER).  But before you focus solely on either SEER or EER; it is even more important to make sure that your new unit is sized properly for your home (only possible by having a Manual J heat load calculation) and that your duct system is sized to match the system that your home needs.

A 16 SEER system installed on a duct system that is too small may only give you 10 SEER.  So saving $1500 on your new unit just cost you $800 per year in additional power bills – for the life of the system.  And that life will be shortened by the extra work it takes to cool the home due to the substandard duct system, costing you even more money.  Make sure that you hire a professional AC Contractor to survey your home and guarantee performance of your new AC system.   The home survey should include your expectations of indoor temperature and humidity settings as that must be factored into the Manual J heat load.

Is the rated SEER of a system totally bogus?

 The SEER rating will give you a rough idea of how efficient the system will be compared to other systems.  But the more the actual conditions you are using it at differ from the test conditions, the less accurate it will be.

Let’s say for example that you keep your thermostat set at 70 degrees.  That’s 10 degrees colder inside than the 80F temperature the system was tested for.  Here’s the problem with that:

All systems of the same Btu/h rating and SEER rating will NOT deliver the same Btu/h of cooling when the indoor temperature is 70 degrees.  And the same thing applies to the outdoor temperature too.  And it also applies to the amount of airflow the blower system is capable of delivering.  If that amount of airflow is less than the amount the system needs to deliver the proper Btu/h and efficiency, you won’t get EITHER the rated Btu/h or the rated efficiency at any indoor or outdoor temperature (ie:  undersized duct system).

 In short:

The Btu/h and SEER rating of a system is only good for the conditions it was tested at.

Let’s keep in mind that the manufacturers know full well that it would be foolish not to tweak their systems to look the best at the SEER testing conditions (80F with 50% RH indoors and 82F outdoors).

Trust 4 Seasons Air Conditioning & Heating, Inc., your heating and air conditioning professional, to install the proper size system that will deliver the best efficiency and the most Btu/h at the conditions found in your climate, in your home, at your chosen indoor temperature preference AND within the confines of the level of restriction to airflow of your home’s ductwork system.

*Photo from www.asm-air.com

Tom Roberts