EER or Energy-Efficiency Ratio is an energy performance rating for cooling devices. You will find them in any air conditioner specification sheet. It’s invaluable when you try to compare different air conditioners as far as energy-efficiency is concerned.

*What does EER mean?*

By definition, it provides you with a ratio of **useful cooling output** *(in BTU/h)* to **electricity input** *(measured in W)*. Higher EER rating means that an air conditioner will be able to provide a lot of cooling effect for every Watt of energy you provide.

We’ll look at how EER is calculated and see what is a good EER rating for different types of air conditioners.

## How To Calculate EER Rating Of An Air Conditioner

Calculating the EER rating is very simple. You need to know only two practical metrics:

**Capacity**of an air conditioner.*Examples: 6,000 BTU, 10,000 BTU, 12,000 BTU, 18,000 BTU.***Power**of an air conditioner.*Examples: 800W, 1,000W, 1,200W, 1,400W.*

To get an EER rating, you just need to divide the capacity of an air conditioner with its power. Here’s the EER equation to use:

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Let’s say we have a 12,000 BTU mini split air conditioner that is powered by 1000W. We can calculate the EER rating like this:

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EER rating of 12 simply tells us that for every 1W of energy we provide to the air conditioner, the air conditioner will give us 12 BTU of cooling effect.

That’s much better than let’s say a 12,000 BTU 1,400W air conditioner. EER rating, in this case, would be 12,000BTU/1,400W = 8.57. That means that for every 1W of energy, this AC would provide us with only 8.57 BTU of cooling effect instead of 12 BTU. In turn, this makes the EER 12 unit 40% more energy-efficient than EER 8.57 unit.

12 EER rating is a very good energy-efficiency rating for an air conditioner. Some of the best devices can even reach more than 12 EER. The best mini split air conditioner Mitsubishi MZ-GL12NA has an extremely high 13.0 EER rating.

It is, however, important to understand that EER is based on a fixed set of conditions, namely:

- Outdoor temperature:
**95F** - Indoor temperature:
**80F** - Relative humidity:
**50%**

Obviously, in real-life conditions, different air conditioners will perform differently. That’s why additional metrics such as SEER and CEEP have been introduced. EER is mostly used for room air conditioners, SEER for central air conditioners and CEEP for window air conditioners.

## What Is a Good EER Rating For AC Unit?

Some types of air conditioners are more energy-efficient than others.

The simplest way of checking which ones are a smart choice if you want to see some electricity bill savings is to check their EER rating.

EER rating is most commonly used when comparing room air conditioners. The most popular of these are portable air conditioners; the ones that can be easily moved from one room to another.

The average EER rating of portable AC units is about 8.5. That means buying ones below EER 8.5 is not a smart move and buying those above 8.5 is quite a smart move efficiency-wise. The most energy-efficient portable AC unit (Whynter Dual-Hose) has an EER rating of 11.2.

Here is a chart of how energy-efficient * portable air conditioners* are (in %) compared to a standard EER 8.5 portable AC unit:

As you can see, an EER 7 unit, for example, is 18% less efficient than the EER 8.5 unit. That means it will spend 18% more electricity for the same effect.

On the other end of the spectra, we see that an EER 11 unit is 29% more efficient than an EER 8.5 unit. In comparison, it will be 29% less costly to run an EER 11 portable air conditioner.

### Theoretical Maximum EER (Based On 1st Law Of Thermodynamics)

In practice, EER rating is calculated by diving the capacity of an air conditioner with the input power, as we’ve seen above. However, by applying the first law of thermodynamics, we can calculate the maximum EER a perfect air conditioner could have.

Theoretical EER derives from the COP ratio (Coefficient Of Performance; A basic metric for cooling and heating devices). Here is the equation that calculates the maximum EER from COP:

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where T_{hot} is the hot summer temperature outside and T_{cold} is the cold temperature that we want the air conditioner to provide inside our house.

Let’s say we have a really hot summer. The outside temperature is 95F (T_{hot} = 308K) and we want to cool down to 80F (T_{cold} = 300K). To calculate the theoretical maximum EER for these conditions we have to plug the temperature (in degrees Celsius) in the upper equation:

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Obviously, the EER rating in real-life can be even 10 or more times lower.