What do the different efficiency ratings mean?

In the United States, efficiency ratings for air conditioners are defined by the Air-Conditioning, Heating, and Refrigeration Institute (AHRI).

These efficiency ratings determine how well the unit will perform in regard to how hard the unit must work to heat or cool the air going into the space. These efficiency values are important to consider when purchasing a unit. The higher the rated value the lower the operating costs for the unit will be. 

In the United States, HVAC (Heating Ventilation and Air Conditioning) equipment will have ratings listed on the unit nameplate, such as “15 SEER”.   

These energy calculations were defined and standardized by AHRI with the assistance from the Department of Energy for all HVAC manufacturers to follow. These ratings ensure consistency throughout the industry, allowing consumers to easily compare efficiency when selecting a unit.  

For a standard Air Conditioner, the efficiency ratings listed will be SEER, IEER, or EER.   

But what do these acronyms mean? 

EER – Energy Efficiency Ratio. EER is a rating used to measure the amount of energy an HVAC unit is consuming to cool the space. This is calculated by dividing the cooling capacity (BTUs per hour) from the Total Energy Input (watt/hr.) – but at a single peak temperature condition. It is effectively a measurement of how efficiently an HVAC system converts electrical energy to cooling capacity at full load cooling conditions. This rating will be especially important for a unit that is to be installed in a warmer climate, where it may be running the majority of the time. Another example would be a window unit. EER is important to know at part load conditions as well to help calculate the overall SEER for the unit. Currently, the industry has adopted using EER2 to give a more accurate measurement of EER throughout the part load run-times.  


SEER - Seasonal Energy Efficiency Ratio. SEER is a ratio that represents the total cooling output (BTUs) of an AC system or heat pump over a cooling season. SEER is a measurement for units less than 65,000 BTU/hr capacity. This is calculated by dividing the number of BTUs (British Thermal Units) removed by the Total Energy (watt/hr.) consumed during a cooling season. This ratio considers part loads conditions of the unit and utilizes EER in its calculation. Calculating this value requires knowing the run time of the unit and EER at part load conditions during a set number of hours during each season. A higher SEER value means a unit can provide a greater amount of cooling for each unit of electricity consumed. Currently, the rating is now SEER2, which has simplified the way SEER is calculated. 

It was designed to show the energy consumption of an air conditioner during an average cooling season period the unit may experience. A SEER rating would be more important for a consumer looking to see how efficiently their unit would operate during different part load conditions during a cooling season throughout the year. This value becomes important when comparing different models and brands that best fit the needs of the consumer for the application. For warmer climates, this would be especially important.   


IEER – Integrated Energy Efficiency Ratio. IEER is used for large commercial units with a full load cooling capacity greater than 65,000 BTU/hr. It represents the integrated or average energy efficiency of a commercial HVAC system at various operating loads over an entire year. Unlike SEER, which focuses on a single set of conditions for multiple “bin” temperatures throughout the year, IEER considers the system’s performance across multiple temperature conditions for a more optimal average that occurs during real-world operation. The calculation for IEER utilizes the EER values at different temperature conditions and the average percentage of when the unit will most likely run in cooling mode at those conditions throughout the year. The formula is as follows and is simpler than calculating SEER:  

IEER = (0.02 * A) + (0.617 * B) + (0.238 * C) + (0.125 * D) 


A = EER at 100% net capacity at AHRI standard condition (95°F) 

B = EER at 75% net capacity and reduced ambient (81.5°F) 

C = EER at 50% net capacity and reduced ambient (68°F) 

D = EER at 25% net capacity and reduced ambient (65°F) 

For a consumer, this would provide a more realistic expectation of energy savings you would experience during operation.  


COP – Coefficient of Performance. COP is used to measure the energy efficiency of a system in heating mode. In other words, it measures the heating output a system produces for a space. COP is a watt/watt measurement taking account of the capacity of the system and the power used. If a unit were to use fully electric heat, its COP would essentially be 1 because it would be 100% efficient.  

COP becomes especially important when using a heat pump because a heat pump is very efficient and will almost always have a COP greater than 1. A higher COP in heating mode indicates that the system can provide more heat output for each unit of energy input, signifying greater energy efficiency. 


HSPF – Heating Seasonal Performance Factor. HSPF is a ratio that measures the heating efficiency throughout an entire heating season. HSPF quantifies how effectively a heat pump converts electricity into heat for heating by considering the total heating capacity in BTUs and the total electrical energy used measured in watts. This is an important value to look at when comparing different models and brands of equipment, especially heat pumps. The calculation process for this ratio is similar to SEER, except this considers performance degradation during the heating season versus the cooling season.   

HSPF is a critical metric for evaluating the energy efficiency of heat pumps in their heating mode. It considers real-world conditions and provides consumers with information to make informed choices about heating systems, promoting the use of more efficient and environmentally friendly technologies. 


SEER2, EER2, HSPF2 - Each energy rating is essentially the same as this article's definitions. However, the “2” on these ratings was adopted to indicate a more accurate way of measuring efficiency. The values may not necessarily be higher values than their predecessors. For instance, SEER2 values will typically be a little less than SEER values even for the same unit model. The reason is the SEER2 value now takes into account a new blower test value into its calculation.  

The main difference with these new values is the way in which they are calculated, taking into account more environmental factors that can occur. This still gives a more accurate value to achieve the highest and most accurate efficiency ratings for the consumer to look at when selecting a unit for their application.