Window-mounted air conditioners (also called room air conditioners) aren’t particularly efficient; the best available models have an EER of about 10 or 11. Central air conditioners (also called whole-house air conditioners or split-system air conditioners) are significantly more efficient; it’s possible to buy one with an EER of 14 or even 15.
So if you care about energy efficiency, you should use a central air conditioner, not a window air conditioner — right? Well, not necessarily.
Air conditioners use a lot of electricity
According to the American Council for an Energy-Efficient Economy, the electricity used for home air conditioning represents almost 5% of all the electricity produced in the U.S. So if you are going to air condition your house, it would appear to make sense to buy the most efficient unit you can find.
There are two metrics used to rate the efficiency of central air conditioners: the Energy Efficiency Ratio (EER) and the Seasonal Energy Efficiency Ratio (SEER). The unit for both metrics is Btu/W•h.
EER is the cooling capacity of the appliance (in Btu/h) at an outdoor temperature of 95°F divided by the current draw of the appliance in watts.
SEER is the total cooling output (in Btu) over the cooling season divided by the total electrical energy input (in watt-hours) over the cooling season. For typical residential air conditioners, EER equals about 0.875 SEER; however, this ratio varies somewhat for different air conditioner models.
EER and SEER ratings
Unlike central air conditioners — which are rated by both EER and SEER — room air conditioners (window units) are rated only by EER.
Room air conditioners that use at least 10% less energy than the federal standard are eligible for an Energy Star label. When shopping for a room air conditioner, look for an Energy Star unit with an EER of 10.7 or more.
The idea behind the SEER rating is to allow consumers to compare the seasonal efficiency (rather than the peak efficiency) of split-system air conditioners. If you are in the market for a new central air conditioner, look for an Energy Star unit with a SEER of at least 14.5 and an EER of at least 12.
Some critics have noted that air conditioner manufacturers design equipment to achieve a high score on the laboratory SEER test, even when some of the engineering solutions used to achieve high test scores result in poor performance or efficiency in the field. For more information on perceived flaws in the SEER standard and SEER testing protocols, see SEER Ratings Challenged.
John Proctor, the president of Proctor Engineering Group in San Rafael, California, has performed useful research into possible flaws in the SEER rating method. Proctor (with co-author Gabriel Cohn) has publshed a paper on the topic: Two-Stage High Efficiency Air Conditioners: Laboratory Ratings vs. Residential Installation Performance. Proctor and Cohn wrote, “The increased installation of high Seasonal Energy Efficiency Ratio (SEER) air conditioners along with utility program rebates for these units prompted a study of the measured performance of these systems. This project assessed the performance of these systems in the climate zones found in the mid-Atlantic region of the U.S. … The data were analyzed to assess the relationship between laboratory testing and real world performance. This study found causes for concern including: actual seasonal energy efficiency ratios between 59% and 84% of the rated SEERs, constant fan operation substantially degrading seasonal efficiencies and reducing dehumidification, latent loads that exceed Manual J estimates, and sensible loads substantially lower than Manual J estimates.”
According to some experts, problems with the SEER rating system may make EER a more useful metric for comparing the efficiency of central air conditioners.
Window units versus central air conditioning
Even though central air conditioners are more efficient than window-mounted air conditioners, homes equipped with window-mounted air conditioners use less energy for cooling than homes with central air conditioning.
For some readers, this fact will be unsurprising. After all, homes with central air conditioning usually maintain lower temperatures in more rooms than homes with window-mounted air conditioners.
Other readers — including those who assume that equipment efficiency is the most important factor in energy use — may be surprised to learn that homes with window-mounted air conditioners are energy misers.
For those interested in the data behind this question, I recommend a blog by fellow energy nerd Tim Grejtak, “The Fascinating World of Air Conditioner Efficiency.” According to Grejtak’s analysis, “Window ACs use less energy on a per cooling degree day and per square footage basis by a factor of 1.75.”
So why do homes with window-mounted air conditioners use less energy for cooling? There are several reasons:
- People living in homes with central air conditioning tend to keep every room in the house cool — even unoccupied rooms — while people living in homes with window-mounted air conditioners are more likely to just cool one or two rooms.
- Central air conditioners often have ducts located in unconditioned attics. Research has shown that air conditioning systems with ducts in unconditioned attics have duct losses amounting to about 20% of the air conditioner’s cooling output.
- Since window air conditioners are often noisy, homeowners remember to turn them off when they leave a room. By contrast, some homeowners with central air conditioning often leave their air conditioning system on all day, even when the home is unoccupied. As explained in an article by Brian Palmer, “The quiet and unobtrusive functioning of central air conditioners … can lead to accidental overuse.”
The bottom line
There are several valuable lessons arising from this analysis:
- Efficient equipment is no bargain if it encourages waste.
- Two or three window-mounted air conditioners cost less to install than central air conditioning.
- Bad ductwork is the Achilles’ heel of central air conditioning systems. If you have central air conditioning, be sure to seal all of the seams in your ductwork and insulate ducts well.
- If you are designing a new home, be sure to locate all ductwork inside of the home’s conditioned envelope.
- If you aren’t home, turn off your air conditioner.
- When you’re sleeping, you may only need a 6,000 Btu/h air conditioner to cool your bedroom. It’s wasteful to use a 2-ton central air conditioner for such a small load.
- If you use one or two window-mounted air conditioners instead of central air conditioning, pat yourself on the back. You probably have a lower carbon footprint than your neighbors who have high-SEER central air conditioners.
- The most efficient way to air condition your house is probably with ductless minisplit units. The only problem with this approach is the high cost of the equipment. However, if you have a new superinsulated house, and if ductless minisplit units can provide both heating and cooling, then these units may be a good, low-cost solution.
- If you live in a dry climate, consider using an evaporative cooler to cool your house. The cost to operate an evaporative cooler is only a fraction of the cost to operate an air conditioner.
Remember: you don’t need a slash
I’ll end with a gentle reminder: the correct abbreviation for “air conditioner” is AC (or A.C.). It is not A/C. There is no slash.
It isn’t a fraction. No one is dividing A by C. It’s an abbreviation for two words. So, please — don’t include a slash.
Last week’s blog: “Choosing an Energy-Efficient Refrigerator.”