IEER Explained: Integrated Energy Efficiency Ratio (vs EER, SEER)

In HVAC, we have several metrics that denote the energy efficiency of air conditioning units. These include EER, SEER, CEER ratings. The newest – and the most accurate – addition to the family is the IEER rating. The IEER rating is slowly replacing the old EER rating and even the newer SEER rating.

Here is a quick history of these energy efficiency ratings for AC units:

  • EER rating is was introduced in the 70s. It’s the most basic and practically fairly inaccurate metric mostly used in portable and window air conditioners.
  • SEER rating was introduced in 2008 as AHRI’s standard. It’s a more measured metric that partly takes into account real-climate conditions (it accounts for part loads and is based on the EER rating). We primarily use it for estimating the energy efficiency of mini split AC units and central air conditioners.
  • IEER rating is the newest rating introduced in 2015. It ‘integrates’ (hence the name) for both part loads (based on EER) and realistic conditions (measurements at different outdoor temperatures). It is thus far the most comprehensive real-climate energy efficiency rating for estimating the energy efficiency of AC units.

IEER stands for Integrated Energy Efficiency Ratio. This is the new energy efficiency rating introduced in 2015’s AHRI Standard 340/360. It is a brilliant metric that built’s on top of EER and SEER rating by introducing a more realistic testing standard for determining the actual energy efficiency (and thereby running costs) of modern air conditioners.

The whole AHRI document titled “2015 Standard for Performance Rating of Commercial and Industrial Unitary Air-conditioning and Heat Pump Equipment” is 68 pages long and quite complex. Our goal here is to simplify and explain what IEER actually means (without too complex equations) and to see how IEER compares to EER rating (EER vs IEER) and to SEER rating (SEER vs IEER).

At the end, we will also show how you can convert EER to IEER and SEER to IEER rating, and visa versa. Note: Exact conversion is impossible; the best we can do is use the rule of thumb approximation.

What Is IEER Rating?

IEER rating goal is to encapsulate (to the fullest extent) the real-time efficiency of AC units. IEER is measured by accounting for two factors that none of the other energy efficiency ratings for air conditioners factor in, namely:

  • Different loads of AC units (part loads include 25%, 50%, and 75% loads; full load (100%) accounts for only 2% of the total IEER rating. The SEER rating, for example, does account for these part loads.
  • Different temperatures (performance of air conditioning units is measured at 95°F, 81.5°F, 68°F, and 65°F outdoor temperatures). The SEER rating fails to account for these different temperatures (it is only measured at 95°F).

The formula used to calculate the IEER rating incorporates (or better still “integrates”; hence the name Integrated EER) both of these factors. Here is what the IEER equation looks like:

IEER = (0.02 × A) + (0.617 × B) + (0.238 × C) + (0.125 × D)

If you think this is a complex equation, don’t worry. We will explain how this calculation is done further on.

To truly understand the importance of the integrated energy efficiency ratio, we first need to look at what EER and SEER ratings tell us. Only understanding these two ratings will give us a clear insight into why the IEER rating was introduced.

EER Vs IEER (Ideal Conditions; 80°F, 95°F, 50%)

The IEER is miles better than the EER. EER is the most basic metric for determining the energy efficiency of air conditioning units. The EER rating is a measurement of AC efficiency and is calculated by dividing cooling output by maximum wattage at specific test conditions. This is what the EER formula looks like:

eer vs ieer rating

Basically, we are measuring the wattage and the BTU capacity at these test conditions:

  • Indoor temperature of 80°F.
  • Outdoor temperature of 95°F.
  • Relative humidity levels of 50%.

Of course, we use air conditioners even when the outdoor temperature is higher or lower than 95°F. The EER rating (as opposed to the IEER rating) measures the performance at ideal (but not realistic) conditions.

This is exactly where IEER and EER differ; IEER is measured at realistic conditions, not perfect ones.

The first attempt to better estimate the energy efficiency (and thereby energy usage) of AC units is the SEER rating:

SEER Vs IEER Rating (Part Load + Full Load At Ideal Conditions)

If you look at how SEER vs IEER ratings are calculated, you may notice that the equations are quite similar. However, there is a big difference between SEER and IEER.

Namely, the SEER builds on top of the EER rating (you can read about what is the difference between EER and SEER here).

The EER rating is measured at 100% full load only. The SEER rating, however, is measured at 25%, 50%, and 75% part loads as well (we talk about EER25%, EER50%, and EER75% in addition to full load EER100%). Nonetheless, even SEER rating is only measured at ideal conditions (80°F, 95°F, 50%).

To calculate SEER rating, you need to make a weighted average of partial and full loads (quite similar to IEER rating). Here is what the SEER rating equation looks like:

ieer vs seer rating

Now, when comparing SEER vs IEER, it is important to understand two things:

  • Yes, the SEER rating does account for different loads for air conditioners.
  • No, the SEER rating does not account for different test conditions.

In comparison, the IEER rating accounts for both different loads as well as different test conditions. That’s why the IEER rating is a better estimate of the actual energy efficiency of AC units than the SEER rating.

To illustrate how the IEER accounts for both of these factors, let’s look at how the IEER rating is calculated:

How Is IEER Rating Calculated?

The SEER rating calculates the energy efficiency at 4 different loads (25%; 95°F, 50%; 95°F, 75%; 95°F, and 100%; 95°F). You may notice that the loads change, but the temperature – outdoor temperature SEER is measured at – stays the same (95°F, to be exact).

Now, the outdoor temperature in the real world is not always 95°F. To account for that, the IEER rating was introduced. This integrated energy efficiency ratio also measures energy efficiency at 4 different loads. On top of that – and this is the key – the measurements are made at different temperatures.

Here are the test conditions for IEER as laid out in Table 6 of 2016 AHRI Standard 340/360 (AHRI also notes that the load measurement can have up to 3% uncertainty):

  1. 25% load at 65.0°F. This is denoted as ‘D’ in the IEER formula.
  2. 50% load at 68.0°F. This is denoted as ‘C’ in the IEER formula.
  3. 75% load at 81.5°F. This is denoted as ‘B’ in the IEER formula.
  4. 100% load at 95.0°F. This is denoted as ‘A’ in the IEER formula.

We see that we both have different loads and different outdoor conditions. Understanding these circumstances, we can look at how the IEER is calculated:

IEER = (0.02 × A) + (0.617 × B) + (0.238 × C) + (0.125 × D)

This is a similar weighted average equation we have seen in the SEER calculation. Basically, it tells us that the IEER rating consists of 4 factors with an attempt to describe how the performance of air conditioners in real-world conditions with higher accuracy. These factors are:

  • 12.5% of the total IEER consists of the measurement of energy efficiency at 25% load and 65.0°F outdoor temperature.
  • 23.8% of the total IEER consists of the measurement of energy efficiency at 50% load and 68.0°F outdoor temperature.
  • 61.7% of the total IEER consists of the measurement of energy efficiency at 75% load and 81.5°F outdoor temperature.
  • 2% of the total IEER consists of the measurement of energy efficiency at 100% load and 95.0°F outdoor temperature.

To illustrate how to calculate IEER rating, let’s look at one example:

Example Of IEER Calculation

Let’s say we have a 14,000 BTU portable air conditioner with an 11.0 EER rating. This is EER rating is the measurement at 100% cooling output. To calculate the realistic-conditions IEER rating, we need to measure the performance at 4 different loads and 4 corresponding test conditions as we have seen earlier on.

Here are the theoretical measurements for this portable air conditioner:

IEER Eq. Symbol: Outdoor Temp.: Load: EER:
A 95.0°F 100% 11.0
B 81.5°F 75% 13.9
C 68.0°F 50% 18.4
D 65.0°F 25% 19.1

Now that we have all the variables in the IEER equation (A, B, C, and D), we can just plug them into the formula and calculate the IEER rating like this:

IEER = (0.02 × 11.0) + (0.617 × 13.9) + (0.238 × 18.4) + (0.125 × 19.1) = 15.6

That means that this 11 EER portable air conditioner has a 15.6 IEER rating.

Now that we understand how the IEER rating is calculated, let’s look at how you can convert IEER to EER and SEER:


We know that EER can be converted to SEER and SEER can be converted to EER. We can make this conversion because both of these ratings are measured at the same test conditions.

If you try to convert EER to IEER (or IEER to EER for that matter), you can quickly realize it’s impossible. That’s because the IEER rating is measured at different test conditions (not only 95°F, but also at 81.5°F, 68.0°F, and 65.0°F).

It’s a bit like comparing apples to oranges. Nevertheless, as both apples and oranges are fruits, both EER and IEER are metrics that determine the energy efficiency of air conditioners.

Therefore we can at least make an approximation of EER to IEER conversion and IEER to EER conversion.

In the solved example above, we have a portable AC unit with an 11 EER rating has a 15.6 IEER rating. In general, we can use a rule of thumb to convert IEER to EER like this:

EER = 0.7 × IEER

And EER to IEER like this:

IEER = 1.43 × EER

Example of EER to IEER conversion: A window air conditioner with a 10 EER rating has approximately 14.3 IEER rating.

Example of IEER to EER conversion: A window air conditioner with a 16 IEER rating has an 11.2 EER rating.

You have to be aware that this is not an exact conversion (because it’s impossible to exactly convert EER to IEER and IEER to EER). It is just a rather good approximation we can use to get a feeling of how energy efficient a certain air condition is.

Similar is true for the SEER vs IEER relationship:


The measurement of the SEER rating is also conducted at certain test conditions. That’s why it cannot be directly converted to the IEER rating since this rating is measured at different test conditions.

The best we can do to convert IEER to SEER and SEER to IEER is to use the approximation.

We already know that EER is equal to about 0.7 IEER. There is also an established relationship between SEER and EER for low SEER ratings (below 14 SEER). This rule of thumb states that EER is equal to 0.875 SEER.

Combining these two principles, we get this rule of thumb for IEER to SEER conversion:

SEER = 0.8 × IEER

If we turn this around, we get an approximation of how to convert SEER to IEER like this:

IEER = 1.25 × SEER

Example of IEER to SEER conversion: A 18 IEER air conditioner has a 14.4 SEER rating.

Example of SEER to IEER conversion: A 16 SEER air conditioner has a 20 IEER rating.

All in all, we hope that you now have a better understanding of what the IEER rating stands for. Chances are that we are going to see more of IEER and less EER and SEER in the future when inspecting air conditioner specification sheets.

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