Measuring cooling efficiency: EER and SEER

Measuring cooling efficiency: EER and SEER

In the last post, we looked at the coefficient of performance (COP) a measure of heating efficiency for devices like heat pumps. COP works well to measure the efficiency of a heating system, but it doesn’t work so well as a measure for cooling systems. Two different measures – Energy Efficiency Ratio (EER) and Seasonal Energy Efficiency Ratio (SEER) are used to compare the efficiency of cooling devices.

Cooling efficiency requires a different equation

To get to the EER, we look at the output cooling energy (in British Thermal Units – BTU) over the input electrical energy in Watt-hours. The resulting ratio – EER – is expressed in BTU.W(1).h(1). It’s a strange unit, but it measures the energy used by the air conditioner over the period of 1 hour.

The equation also needs to take some other factors into account, one of which being the “tons” of air conditioning provided by the unit. It gets even stranger, because EER is usually calculated using specific inside and outside temperature, humidity and dew point conditions. In other words, the EER you see on a device describes the way the device will work under very specific conditions – which are rarely what Mother Nature happens to whip up.

Even though EER can’t really predict how much an air conditioner will cost to operate, it is still useful because it gives us a way to compare units – even under “ideal” conditions that may never actually materialize. The higher the EER rating of a unit, the more efficient it will be.

The likelihood that a unit operates at the ideal conditions for most (or even any) portion of the summer is pretty small. Another way to look at cooling efficiency is measuring it over the course of a season, when natural variations in temperature can be taken into account. Rather than looking at the way the unit performs under ideal conditions for one hour, the SEER tries to describe the way the unit will work over an entire season, complete with the natural seasonal variations in temperature.

Early and late in a cooling season, the unit may only be turned on for a few hours per day. On most days of the season, the unit may be on for 8-10 hours, and on a few days, (unless you live in a place like Tucson, AZ), the unit may be running 14-16 hours per day. The SEER formula tries to balance out the amount of energy the unit uses over the course of an entire season, when operating conditions vary remarkably from month to month.

Even with an adjustment for seasonal variations in outside temperature, it’s still difficult – maybe impossible – to come up with an accurate assessment of cooling efficiency during a cooling season.

So, if the EER and SEER aren’t very good predictors of energy consumption for installed units, why do we use them? We use them to compare units to each other, to determine which units operate more efficiently under a given set of circumstances. A unit with a SEER of 14 is more efficient under the given conditions than a unit with a SEER of 13 is under the same conditions.

No matter what actual conditions you operate under, you can tell which unit will be more efficient – even if you can’t tell how much it will cost you. Just as with EER, the higher the SEER, the more efficient the device is.

The federal government always looks at the relative efficiency of heating and cooling devices. Over time, the minimum cooling efficiency requirements for air conditioners have changed, and will continue to change. Manufacturers are required to make new devices operate more efficiently, which ultimately saves money on operating costs.

Heat pumps are exceptionally efficient and produce much higher EER and SEER ratings than conventional central air conditioning systems do. EER and SEER ratings allow you to compare the efficiency of a ductless system to a central air conditioner. You can see that you’ll save money over the course of an hour or an entire season with a ductless system because its EER and SEER ratings are so much higher.

If you’d like more information about the cooling efficiency of ductless air conditioning and how it can save money compared to a conventional central air conditioner, please contact us at New England Ductless at (781) 995-2665. We’ll be happy to show you how you can save money not only on your cooling costs, but also on your heating costs in the winter.

Photo Credit: Ann Kathrin Rehse, via