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…