Understanding EER: Energy Efficiency Rating For AC Explained

EER or Energy-Efficiency Ratio is an energy performance rating for, primarily, cooling devices. You will find them in any air conditioner specification sheet. The energy-efficiency ratio is invaluable when comparing different air conditioners as far as energy-efficiency is concerned.

What is EER?

Definition: EER rating provides you with a ratio of useful cooling output (in BTU/h) to electricity input (measured in W).

A higher EER rating means that an air conditioner will provide a lot of cooling effect for every Watt of energy you provide.

What is EER in AC?

Quick example: We have a 10,000 BTU AC unit with 10 EER. EER rating (10) tells us that we’ll need to provide an air conditioner with 1,000W of power to give us 10,000 BTU of cooling capacity.

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

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 need to divide an air conditioner’s capacity by its power. Here’s the EER equation to use:

EER rating = Capacity (in BTU) / Power (in W)

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:

EER rating = 12,000 BTU / 1000 W = 12

EER rating of 12 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 an excellent energy-efficiency rating for an air conditioner. Some of the best devices can even reach more than 12 EER. The best mini split air conditioners have an extremely high 12+ EER rating (check them here).

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’ energy-efficiency.

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, which 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 advisable 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:

chart of below average and above average energy efficiency ratio rating of portable air conditioner

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.

A special case are the battery-powered portable AC units. For example, the best battery-powered AC Zero Breeze Mark 2 has an EER rating of only 3.54, despite being a dual-hose unit. That is, sadly, the nature of battery-powered electrical devices.

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

In practice, EER rating is calculated by dividing an air conditioner’s capacity 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:

COP = Tcold / (Thot – Tcold)

EER = 3.41 x COP

where Thot is the hot summer temperature outside, and Tcold is the cold temperature that we want the air conditioner to provide inside our house.

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

EER = 3.41 x 300K / (308K – 300K) = 127.9

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

22 thoughts on “Understanding EER: Energy Efficiency Rating For AC Explained”

    • Hello Paulo, to calculate EER you need two numbers; 9000 BTU and power (in Watts). If, for example, the 9000 BTU unit is powered by 1000 W, the EER rating is 9.

    • I will tell you a very simple Procedure to calculate EER (average) air conditioner with capacity of 9000 BTU, firstly we have to to obtain the in put power in watt in 2 options;
      1- air conditioner charged by iso-butane R600a refrigerant , let us to measure the power supply voltage and the full load current while our air conditioner in operation, we have to calculate the in put power in watt P=I*V*pf (single phase power factor) assume the result in watt equal 750 watt , therefore the EER=9000/750=12
      and the second option
      2- air conditioner charged by Carbon dioxide R744 refrigerant , let us to measure the power supply voltage and the full load current while our air conditioner in operation, we have to calculate the in put power in watt P=I*V*pf (single phase power factor) assume the result in watt equal 850 watt , therefore the EER=9000/850=10.58 .
      the average of the EER is 12+10.58/2= 11.29

  1. Why?
    “Obviously, the EER rating in real-life can be even 10 or more times lower.”

    What is the difference EER=3.41xCOP and ERR=(BTU/hr)/W . Why this big slack between?

    • Hello Darwin, the reason is based on thermodynamics. The process of compression and expansion of gases is extremely inefficient in practice. Theoretically, the calculation of maximum EER is based on the 2nd Law of Thermodynamics. When this law is applied to practical heating or, inversely, cooling system, the percentage of input energy that is transformed into useful work drops significantly.

  2. “To calculate the theoretical maximum EER for these conditions we have to plug the temperature (in degrees Celsius) ..”

    I am not getting your point right. Relating EER to COP, measuring Temperature in Fahrenheit, converting it to Kelvin and asking for degrees Celisus?
    I think that is the ideal theoretical figures one must opt for when looking for an A/C which doesn’t exist!
    Interestingly, I cannot see the Inverter technology contribution to any formula that defines an A/C efficiency! Especially that it is electrical power consumption and pollution that we are talking about.
    Thanks for simplifying the case

    • Hello Issam, you’re right, ideal theoretical figures don’t exist in the ideal world. That is an EER equation derived from the 1st and 2nd Law of Thermodynamics; the purpose of presenting theoretical maximum is for academic-inclined people to understand fully how EER calculation is governed by laws of thermodynamics.

      Inverter technology has an effect of EER; namely on the underlying macroscopic metrics such as BTU and Power. The power needed to achieve a high BTU is usually lower with inverter technology thereby the EER is higher.

      • Thanks for your explanation. I live in Dubai where temperatures reach 50 degrees and from my experience i found that Inverter did reduce my power consumption, as well as placing the condenser in area of less direct sun exposure andreducing the distance between it and the Air Handling unit inside the home (pipes less exposed to outside heat).
        I have drifted away from the topic, but i think one should pay more attention to the point that installation is very important to maintain the Efficiency

        • We get 50 C here in Phoenix on rare occasion. I work in HVACR and always advise putting condensers in a shady spot. When I have customers trying to cool their homes to 24 degrees C and pushing 50 degree C outdoor temps and you consider the SEER test conditions of 90-95 degrees those numbers go out the window. I think EER is a better number to go off of here because it is so dry and many people use evap coolers for all but June, July, and August when the monsoon hits. About 7 months of the year you don’t even need AC or Heat so SEER is just kinda useless. Still I am trying to get a better understanding of the actual test conditions for EER.

  3. hello,
    very good article. one thing here which I failed to understand is that in the definition you clearly said, Definition: EER rating provides you with a ratio of useful cooling output (in BTU/h) to electricity input (measured in W). so the it is the ratio of power to power (Btu/h to Watt).However, in the top picture and in the heading, How To Calculate EER Rating Of An Air Conditioner, you defined it as: EER rating is equal to Capacity (in BTU) / Power (in W). So, first you said it is BTu/h to W and then you said it is BTU to W. I think that it should be BTU/hr here, not BTU.

    • Hello there, you’re completely right. The appropriate unit is BTU/hr; it a measure of power – work done in a unit of time (P = A/t). However, in HVAC lingo, we always use BTU (we omit the ‘per hour’). It’s easier to understand.

  4. I am looking at the Frigidaire GHWQ103WC1.

    It claims a CEER (and EER) of 15.

    But it claims to be 10,000 BTU of cooling and to use 890W of power, which by my calculation is 10,000/890 = 11.2 EER.

    Am I missing something, or are they just lying about their EER?

    • Hello Jeff, you are looking at Frigidaire Gallery 10,000 BTU window unit? Yes, they do claim that the CEER rating is 15 and it might as well be. CEER and EER are not the same; CEER is a newer efficiency rating, you can read all about this Combined Energy Efficiency Ratio here.

      Usually, CEER is a bit of an unknown rating. We all understand EER and can compare window AC units based on the EER rating. Your calculation for the EER rating is exactly right: with 10,000 BTU cooling output and 890W power input, you get an 11.2 EER rating. Hope this helps.

  5. Power Input in Watts should be the maximum one needed to be calculated? For eg. If the rated btu of an ac is 19500 and max power input is 2800 W. That means it is generating 6.96 EER. However, the manufacturer claim EER is 4.0.

    Can you please help me with this.

    • Hello there, your thinking is correct. You have 2 inputs in the EER equation, namely specified max. wattage and specified max. BTUs. The ratio is equal to EER rating. That manufacturer-specified EER rating seems to be off.

  6. My contractor wants me to buy two York mini splits, model YHGE12XXMAXA-RX rather than one big unit. All the writing on the boxes are in Spanish which I do not read. What I want to know is what is the EER. However it only lists an SEER. What is an SEER?

    Would I be better off with one large unit instead of two smaller ones?

    • Hi Mike, EER is used primarily for portable and window AC units. For mini splits, you would use SEER rating (stands for Seasonal EER). You can read more about what SEER rating is here.

      1 vs 2 units is basically 1-zone vs 2-zone mini split? So 1 outdoor unit and then 1 or 2 indoor units? If that is the case, the 2 indoor units make sense if you want to cool/heat larger spaces. The temperature will be more even and stable (no hot pockets). If it is a smaller place, 1 indoor unit will suffice. Hope this helps.


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