EER vs SEER: Difference Between SEER And EER Rating

EER and SEER ratings are both metrics that determine the energy efficiency of air conditioners. Since they measure the same thing, we will look into how they differ in the EER vs SEER overview.

Example: A mini split air conditioner has a 19 SEER rating and a 12.7 EER rating. What does that mean?

First of all, let’s look into what EER and SEER stand for and how they are defined:

• EER stands for Energy Efficiency Ratio. It is calculated by dividing AC cooling output (in BTU/hr) with maximal wattage of an air conditioner. EER is used to denote the theoretical energy efficiency of portable AC units and window AC units.
• SEER stands for Seasonal Energy Efficiency Ratio. It is calculated as a weighted average of different EER ratings (EER_25%, EER_50%, EER_75%, and EER_100%). SEER is used to denote the practical energy efficiency of mini split air conditioners and central aircon systems. It was introduced in the 2008 ANSI/AHRI Standard 210/240 directive.

Here is the main difference between SEER and EER ratings:

EER rating is more of a theoretical measure of AC’s energy efficiency. The goal of SEER is to more adequately capture the practical energy efficiency of air conditioners during the summer.

In short, the EER rating formula is a simple calculation. To calculate the EER rating of an air conditioner, we just divide the cooling output by maximum AC wattage like this:

The cooling output is measured at certain EER test conditions: Outdoor temperature of 95°F, the indoor temperature of 80°F, and 50% relative humidity.

This is a theoretical calculation. Practically, however, we use air conditioners at different outdoor and indoor temperatures, and at different relative humidity levels.

To best capture these realistic conditions, the AHRI has introduced the SEER rating. The SEER rating is calculated as a weighted average of different EER ratings. Here is the SEER formula to illustrate that:

Let’s quickly explain what all this weighted average SEER rating means. We will first look at the EER rating and then compare the EER rating to the SEER rating. We will also show you how to easily convert EER to SEER and SEER to EER:

EER Rating Explained

The EER rating is the simplest possible measure of the energy efficiency of air conditioners. The SEER rating is basically just a practical derivative of the EER rating. It is important to understand that EER is measured at specific test conditions by AHRI (Air Conditioning, Heating, and Refrigeration Institute).

AHRI tests portable and window air conditioners at:

• 95°F outdoor temperature.
• 80°F indoor temperature.
• 50% relative humidity levels.

That means that we run these room AC units at these temperatures and relative humidity. To calculate the EER rating, we only need to measure two metrics, namely:

• Power input. This is the electrical power we use to generate the cooling output. AHRI simply measures the maximum wattage draw of the AC at test conditioners. You do this with a watt-meter.
• Cooling output. This is a measure of how much cooling capacity we get from a room AC unit with the measured power input at the EER test conditions.

Based on these two metrics, we calculate the EER rating using this formula:

Example: We are testing portable air conditioners at the test conditions (95°F, 80°F, 50% humidity). The maximum wattage the unit runs on is 1,000W. We now measure that the total cooling output when the AC is running at 1,000W is 12,000 BTU/hr. Basically, we have a 12,000 BTU air conditioner. Here’s how the EER rating is calculated:

EER Rating = 12,000 BTU/hr ÷ 1,000 W = 12

That means that we have a 12 EER rated air conditioner. To get an idea of what a good EER rating for an air conditioner is, here are the average EER ratings of room AC units:

• Average EER rating of portable air conditioners is about 8.5 EER.
• Average EER rating of window air conditioners is about 10 EER.
• Average EER rating of through-the-wall air conditioners is about 9 EER.

Now, in the real world and during summer, we don’t always have 95°F outdoor temperature. Nor do we always have 80°F indoor temperature. Of course, not even the relative humidity levels are 50%.

At different temperatures and relative humidity, the air conditioner will have different performances. In short, the EER rating test conditions are the perfect conditions. If you were to measure the energy efficiency of an air conditioner at let’s say 110°F outdoor temperature, 78°F indoor temperature, and 60% relative humidity the energy efficiency will fall.

That also means that the standard EER rating is not all that good a measure of how well the AC unit will perform in realistic conditions…

… but SEER rating is.

The whole point why the SEER rating was introduced is to better estimate the energy efficiency of air conditioners at realistic summer conditions.

SEER vs EER Rating

The goal of the SEER rating is to more precisely estimate the energy efficiency and thereby cooling costs that the EER rating.

To achieve that, additional tests are done when measuring the SEER rating. The EER rating is normally measured at 100% wattage (full load EER). This is denoted as EER_100% in the SEER equation:

In addition to this EER_100%, AHRI measures the energy efficiency of air conditioners at 25% wattage (EER_25%), 50% wattage (EER_50%), and 75% wattage (EER_75%) as well. These are known as partial load EER ratings.

The final SEER calculation is a weighted average of these 4 different EER measurements.

To illustrate how SEER is calculated based on these 4 EER measurements, let’s look at an example:

Let’s say that we have a 12,000 BTU air conditioner with a 12 EER rating from the previous example. Here are theoretical measurements of energy efficiency at 25%, 50%, 75%, and 100% wattage:

• 4,000 BTU at 25% load (250W). That yields a 16 EER_25% rating.
• 7,500 BTU at 50% load (500W). That yields a 15 EER_50% rating.
• 10,500 BTU at 75% load (750W). That yields a 14 EER_75% rating.
• 12,000 BTU at 100% load (1,000W). That yields a 12 EER_100% rating.

With these partial and full EER loads, we can calculate the SEER rating. Let’s just plug all of these metrics into the SEER equation:

SEER Rating = (1×12 + 42×14 + 45×15 + 12×16) / 100 = 14.67 SEER

As we see, we get an EER vs SEER ratio of 12 vs 14.67. In general, the SEER rating is always higher than the full load EER rating.

SEER rating measures the seasonal energy efficiency precisely enough for EnergyGuide to estimate yearly expected running costs based on the SEER rating.

Now, in a lot of cases, we need to convert EER to SEER and SEER to EER.

For this purpose, we can use 2 equations.

Equation 1: Here is the simplified equation you can use for lower SEER ratings (below 14 SEER).

EER = 0.875 × SEER and SEER = 1.143 × EER

Example: We had 12 EER rated AC units. SEER is calculated as:

SEER = 1.143 × 12 EER =13.72 

We see that this equation doesn’t get us the exact EER vs SEER conversion. It’s a good approximation, however. You can read more about how to convert SEER to EER here.

For a more precise calculation, we need to use Equation 2.

Equation 2: This is a more complex but also more exact SEER vs EER calculation.

EER = −0.02 × SEER² + 1.12 × SEER

We delved into how to convert EER to SEER precisely in this article.

With all this in mind, hopefully, you now have a better understanding of the somewhat complex differences between EER and SEER ratings.

You can also read what specifically EER rating means here and what SEER rating means here. Both of these articles involve cost-saving calculators in US dollars.