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Air Conditioner Efficiency

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Equipment used in cooling systems in residential and small commercial buildings often express the cooling system efficiency in terms like

  • EER - Energy Efficiency Ratio and/or
  • SEER - Seasonal Energy Efficiency Ratio

For air conditioners in rooms it is common to use EER - Energy Efficiency Ratio .

For central air conditioner systems it is more common to use SEER - Seasonal Energy Efficiency Ratio.

These ratings are normally posted on the Energy Guide Label attached to all new air conditioners.

Some air conditioner manufacturers participates in the voluntary Energy Star labeling program where the Energy Star label indicates higher EER and SEER ratings.

EER - Energy Efficiency Ratio

EER is a measure of how efficient a cooling system operates in steady state (over time) when the outdoor temperature is at a specific level (outdoor conditions commonly used are 95 oF (35 oC) ).

  • the higher EER - the more energy efficient is the system

EER can be calculated

EER = q c / E                                              (1)


q c = output cooling energy (Btu)

E = input electrical energy consumption (watt-hours, Wh)

EER is common for room air conditioners ranging 5000 - 15000 Btu per hour (1.5 kW - 4.5 kW) .

  • 1 Btu/h = 2.931x10-4 kW = 0.0299 kpm/s = 0.252 kcal/h = 3.986x10-4 hk = 3.939x10-4 hp = 0.2163 ft lb/s

In mild climates air conditioners with EER of at least 9.0 should be selected. In hotter climates air conditioners with EER above 10 should be selected.

Note that EER is sometimes erroneous based on cooling power and electrical power consumption as

EER power = P c / P w (1b)


P c = output cooling power (Btu/h)

P w = input electrical power consumption (W)


SEER - Seasonal Energy Efficiency Ratio

SEER  - Seasonal Energy Efficiency Ratio - can be calculated

SEER = Q c / E                                                     (2)


Q c = seasonal cooling energy (Btu)

E = seasonal electrical energy consumption (Wh)

SEER should be at least 10 - there are units where SEER reach at least 17 .

Example - EER power vs. COP

A cooling unit operates at 1 ton cooling (1 ton/kW) - or 12000 Btu/h .

The Energy Efficiendy Ratio EER power can be calculated as

EER power = (12000 Btu/h) / (1000 W)

= 12

Coefficient of Performance - COP - can be calculated as

COP = P c / P w (3)


COP = Coefficient of Performance

P c = output cooling power (Btu/h, W)

P w = input electrical power consumption (Btu/h, W)

Since 1 kW = 3412 Btu/h - COP for 1 ton cooling can be calculated as

COP = (12000 Btu/h) / ((1 kW) (3412 Btu/h / kW)

= 3.52

A small cooling unit operating at 1 ton per kW (1000 watts) is equivalent to a COP of 3.52 or an EER power of 12 .

The relationship between EER power and COP can be expressed as

EER power = 12 / 3.52 COP

= 3.41 COP

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Related Topics

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Design of Air Conditioning systems - heating, cooling and dehumidification of indoor air for thermal comfort.

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Calculating Cooling Loads

Calculating chiller and cooling tower refrigeration loads - in tons.

Cooling and Heating - Performance and Efficiency Terminology

Performance and efficiency terminology related to heat pumps and air condition systems.

Cooling Load - Convert between kW/ton to COP or EER

Convert between cooling load units like kW/ton, COP and EER.


The measure of usefulness.


Efficiency is the ratio useful energy output to energy input.

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