All window air conditioners work in the same fashion. They cool by extracting hot and humid air from a room, cooling and dehumidifying it, and returning it back into the room while exhausting warm air to the outside. In more technical terms, a window air conditioner uses its compressor to compress refrigerant into a hot, high-pressure gas. The refrigerant gas then travels through the condenser coil, where it is condensed into a liquid and sent through the expansion valve and over the evaporator coil to absorb heat and cool down. Hot air is exhausted out through the back of the air conditioner – the part facing the outside – while cool air is blown into your room using the air return grille located on the front of the system.
Energy Efficiency Ratings
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Window air conditioners are more often used as the primary source of cooling for small spaces that aren't covered by a central AC. However, it's not unusual for consumers who regularly use central air conditioning units to look to window air conditioners as a way to cool rooms using less energy during the warmer months. These systems typically cost less to purchase initially and less to operate over the long-term if sized correctly for your space. Look for two things when evaluating a unit's energy efficiency:
Typically, every 0.1 increase in EER translates into a 1 percent drop in your electricity usage. To achieve the best energy- and cost-savings, the U.S. Department of Energy recommends that you shop for a room air conditioner that has an EER of at least 10.0. Window ACs manufactured by Fedders are known for both their high EERs and Energy Star seals. |
Energy Star Seal – Energy Star room air conditioners use at least 10 percent less energy than conventional models. These systems are often loaded with useful features to give you greater temperature control and make energy consumption easier to manage. These features include programmable timers, digital thermostats, automatic cooling modes, and multiple cooling speeds. To calculate how much energy and money an Energy Star window AC will save you compared to a conventional model, use 
Energy Efficiency Ratio – An EER measurement is another efficiency indicator for room air conditioners, and it's gaining popularity across the window AC industry. This ratio tells you how many British Thermal Units (BTUs) of heat energy a window AC uses for each watt of power. As a rule of thumb, remember the higher the EER, the more efficient the air conditioner. 