For consumers purchasing a new AC unit, the first place they turn when researching efficiency is to the manufacturers’ SEER rating. The Seasonal Energy Efficiency Ratio is a metric used to measure the ratio of cooling output to energy consumed. In theory, at least, the higher the SEER rating, the more efficient the unit will be.

Unfortunately, SEER ratings don’t give a completely accurate picture of energy efficiency. Much like miles-per-gallon ratings for automobiles, SEER ratings are determined under optimal conditions.

Let’s take a quick look at SEER ratings to understand a little about what they are, and another way of looking at energy efficiency that may be better at determining the best AC choice.

The Problem with SEER Ratings

SEER ratings were developed as a marketing term for the air conditioning industry by the manufacturers that make traditional, compressor-based air conditioners as a way to determine the efficiency of their products.

Unfortunately, SEER ratings do not give a completely accurate picture of energy efficiency. As a baseline, SEER measures performance at 82 degrees. Most units do not need a lot of power to cool a space at that temperature. When you raise the baseline to 100 degrees, which is a much more realistic number in many areas during the summer, you lose a lot of equivalent air conditioning.

By increasing the temperature by 18 degrees, the unit’s cooling capacity decreases while energy consumption increases. So you may think you are getting an energy-efficient SEER rating of 13, but in fact it’s much less.

A Better Solution

A more accurate way to determine efficiency is using the Energy Efficiency Ratio. EER is calculated by taking total BTU of heat rejected, or actual cooling per hour, and dividing it by the watts of electricity used to reject the heat. The main difference is the SEER is calculated for a controlled environment, whereas EER takes seasonal variations into account. That means the EER value can vary depending on the temperature and humidity of where the unit is located.