EER Explained: The Energy Efficiency Ratio in Air Conditioning

Written by: Paul Cathro

Updated on: January 6, 2023

fans and air conditioners

It isn't a coincidence that your energy bills rocket when you use your air conditioning system. The powerful cooling performance of air conditioners requires lots of electricity, making them expensive to run.

Thankfully, there is a way to choose an efficient air conditioner – EER.

EER or Energy Efficiency Ratio indicates a portable air conditioner's energy performance, showing you the cooling output for every watt of energy.

A higher EER rating indicates the air conditioner uses less energy to provide cooling power, so it is more efficient than one with a lower EER rating.

This article covers everything you need to know about EER, starting with the most basic question of them all.

What is EER?

EER (Energy Efficiency Ratio) is a laboratory measurement of the energy efficiency of an air conditioner based on fixed conditions.  

The EER shows the air conditioner's efficiency at a specific temperature, providing a baseline measurement for different temperatures. 

The calculation takes the air conditioner's cooling capacity in British Thermal Units (BTU per hour). It compares it to the unit's power input in watts, telling us how much energy the air conditioner uses at peak cooling power.

A higher EER rating means the air conditioner has a higher cooling capacity per watt, making it more efficient and cheaper to run. 

EER ratings are most common on portable air conditioners, while central air uses CEER. 

Of course, portable air conditioners can consume less or more power than quoted by the manufacturer, and we’ll examine those factors later. 

Why Should I Care About EER Ratings?

A more efficient air conditioner consumes less energy, which saves you money on your energy bills and is better for the environment. 

Without EER ratings, you wouldn't know which air conditioners use less energy, and manufacturers would have no incentive to develop efficient technologies. EER ratings keep innovation alive and make energy efficiency desirable. 

If you are concerned about climate change, you are also probably concerned about your carbon footprint. High-efficiency air conditioners have a lower carbon footprint, so they are better for the environment. That's a win-win for everyone!

How to Calculate the Energy Efficiency Ratio

While some manufacturers calculate EER and use labeling to demonstrate energy efficiency, you might have to do it yourself. The excellent news is calculating EER is easy – all you need is the BTU rating and the unit's wattage. 

To calculate EER, Divide the BTU (British Thermal Units) rating by the wattage.

Here's what the calculation looks like:

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

For example:

  • If you have an 8,000 BTU air conditioner powered by 1,000W, the EER rating is 8. 
  • If you have a 12,000 BTU air conditioner and the power is 1,200W, the EER rating is 10.

These ratings tell us that for every 1W of energy we provide, the air conditioners give us 8 and 10 BTU of cooling power, respectively. 

A higher number is better – so in these examples, the more powerful 12,000 BTU unit uses less energy per BTU of cooling power. 

For example, the dual-hose Whynter ARC-14S has a 10.76 EER rating based on 14,000 BTUs of cooling power and a 1,300-wattage rating. This is considered excellent amongst portable air conditioners which are notoriously power hungry devices .

Factors Affecting the EER

It's important to note that the EER is a baseline figure based on fixed conditions

In real-world use, portable air conditioners can consume more energy, especially in high humidity and after extended downtime periods. 

Factors affecting air conditioner efficiency include:


Higher humidity forces the air conditioner to run on higher settings and work harder, consuming more electricity. 

Undersized system 

If your portable air conditioner is too small, it will run on maximum power without ever cooling down your room sufficiently. 

Oversized system 

A portable air conditioner that is too big will consume more energy than is needed and run shorter cycles, which can make the coil freeze.  

Dual or single hose

In general, dual-hose portable air conditioners are more efficient than single-hose models, using less energy for the same cooling power. 

Poor airflow

Placing a portable air conditioner too close to walls reduces air intake, making it work harder and longer to bring the room down to temp. 

Dirt and grime

Dirty air filters restrict airflow and create air turbulence, limiting cooling output, while dirty coils create cold spots and uneven cooling. 

Duct leaks

A leaky duct or exhaust hose forces warm air back into the room, slashing the air conditioner's cooling efficiency. 

Low thermostat settings

Setting the thermostat lower increases energy consumption by increasing cooling power, knocking EER figures out of the park. 


As portable air conditioners get older, they get less efficient as mechanical components and moving parts wear down. 

Open windows and doors

Open windows and doors invite humidity and warmth into your home, making your portable air conditioner work in vain on high settings. 

Neglectful maintenance 

Portable air conditioners require regular filter changes and cleaning to maintain system performance. 

EER vs. SEER: What's the Difference?

Some air conditioners come with a SEER (Seasonal Energy Efficiency Ratio) instead of an EER rating.

SEER is a more comprehensive measurement because it measures how efficiently an air conditioner operates over an entire season. 

While EER is a standardized energy efficiency measurement with one set of conditions (mimicking cooling performance in summer), SEER is calculated in different climates to indicate real-world energy consumption. 

However, SEER is only typical in central air conditioning systems – portable air conditioners nearly always have an EER rating. This reflects the typical usage of portable air conditioners, which are mostly booted up in the summer. 

Another energy ratio is CEER (Combined Energy Efficiency Rating). This combines the EER and the standby mode power consumption. However, it's typically only used for window and room air conditioners.

What is a Good EER Value for an Air Conditioner in 2023?

Unsurprisingly, some air conditioners are more efficient than others. 

The most efficient portable air conditioners have an EER of over 11, and anything between 8.5 and 9.5 is considered good.  

But what does this mean? 

For every half-point increase in EER, an air conditioner is around 6% more efficient. This means an air conditioner with an EER of 10 is 18% more efficient than one with an EER of 8.5 – simply put, it costs 18% less to run. 

This half-point increase soon adds up. A portable air conditioner with a below-average EER of 7 is 24% less efficient than one with an EER of 9. 

However, it's essential to strike a balance between efficiency and air conditioner price because more efficient models cost more money. 

You can expect to pay 20-40% more for a portable air conditioner with an EER of 11 than one with an EER of 9. Whether the energy savings offset the higher purchase price depends on your usage and how long the air conditioner lasts. 

The most efficient portable air conditioners are dual-hose models, which benefit from a more efficient air exchange process. Dual-hose air conditioners cool an area more quickly while consuming less energy. 

When shopping for a portable air conditioner, look for models with an EER over 9, and if you want ultimate efficiency, look at dual-hose models.

About the Author Paul Cathro

Paul is an ex-HVAC engineer with 5 years 'in the trade'.

He acquired in 2022 and aims to make it the internet's most comprehensive HVAC resource for small homes in the next few years.

You can learn more about Paul's story here.

Browse his published work on the website here.

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