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EER in Air Conditioners: What is it & How to Calculate

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Air conditioning units are becoming increasingly common and needed in UK homes as the summers get hotter and most homes are built to contain heat, rather than keep us cool. Before buying domestic air conditioning though, it’s important to consider its efficiency to avoid seeing your energy bills skyrocket along with your household’s carbon footprint. 

EER, standing for Energy Efficiency Rating, measures the efficiency of an air conditioner in a property. A handy metric that families and businesses can use when considering having air conditioning installed, it allows you to shop around and find the most efficient unit for you that will save money long term. 

What is EER?

  • EER = Energy Efficiency Ratio

The ratio looks at the cooling capacity of an air conditioning unit (in British thermal units, BTU per hour) compared to the power input of the unit (watts), to measure how much heat energy the unit can remove. It’s a ratio usually used for indoor, room air conditioners like split system air conditioning units


  • Cooling capacity (BTU per hour): Power input (watts)

Cooling energy is always measured in BTU, which stands for British thermal units, and power is measured in watts. BTU outlines how much heat energy the unit can remove from the air, hence this sums up its cooling capacity. 

Essentially, the higher an air-con unit’s EER rating, the more efficient it is due to it having a higher cooling capacity per watt of power. 

Calculating Energy Efficiency Ratio

If EER is not provided when shopping for ai conditioning, you can easily calculate it yourself, all you need is the unit’s wattage and BTU values which you can usually get from the manufacturer if not provided by your potential supplier. 


For an accurate and baseline EER calculation, the external temperature should be consistent at 35 degrees celsius, the external temperature should be 26 and the humidity levels should be at 50%. This way, EER’s across units can be compared fairly and reliably. 

Although EER is a reliable and valuable way to determine the overall efficiency of an air conditioner for your home, you should be aware that the actual efficiency can vary due to various external factors like weather and humidity. The bigger the difference in temperature between indoors and outdoors, the harder your unit will have to work and the less efficient it will be able to run. 

What’s a Seasonal Energy Efficiency Ratio (SEER)?

In countries where the climate changes drastically between seasons (like here in the UK), many manufacturers and suppliers will also use Seasonal Energy Efficiency Ratios (SEER). This is similar to EER but takes into account seasonal factors and air conditioning usage rather than rating a unit’s efficiency all year round. 

SEER is a lot more specific and granular by calculating the cooling capacity (BTU per hour) and power input (watts) but only throughout the key cooling period as opposed to all year, which for us in the UK would mean summer, albeit a very short one. 

Why should you prioritise EER when buying split system air con?

It’s hugely beneficial to shop for an air con unit using EER and SEER ratings rather than just going off their cooling capacity (BTU). For example, a unit may have a strong capacity to cool your home but if it uses little power to do so it can end up costing you tons in energy bills and will increase your carbon footprint which is not something you want. 

Some of the benefits of prioritising EER when deciding which air conditioner to buy are:

  • Lower carbon footprint
  • Higher performing system
  • Higher quality and more reliable system
  • Reduced energy bills

What’s the best EER for an air conditioner?

Generally, an EER rating of 12 or over is classed as desirable, though when comparing multiple air conditioners, always aim to go for the highest rated unit within your budget. Many split system air conditioning units on the market now have EER’s above 12 too, making it easier for household’s to reduce their energy usage.

Need a new air conditioning unit?

BOXT’s entire range of split system air conditioners is rated highly for energy efficiency, to take the stress out of your decision and ensure households across the UK are reducing their carbon footprint. We don’t stock any air conditioners that will end up costing you a fortune in energy bills. You can answer a few simple questions about your home to discover a range of units that are suited to your property type, size and requirements.

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