What Is 9000 BTU Coverage?
A window air conditioner rated at 9,000 BTUs (British thermal unit) can theoretically cool 350 to 400 square feet, according to Energy Star. But that rating is merely a projection. Many factors affect the cooling capacity of your air conditioner, including the home's insulation, age of the windows, number of occupants, which room the air conditioner cools and whether the home is shaded or exposed to direct sunlight.
What Does the BTU Rating Mean?
The Best Air Conditioner Review website explains that a BTU is the unit of energy required "to raise the temperature of one pound of water by one degree Fahrenheit." Applied to air conditioning science, an air conditioner functions by lowering the temperature of water in the air. Because cool air holds less moisture than warm air, the water condenses in the air conditioner and is drained outside the window.
Choosing the Best Size Air Conditioner
To calculate the best number of BTUs in an air conditioner, EnergySavers.gov recommends 20 BTUs for each square foot of living space. In a square room with an area (length times width) of 100 square feet, you would need 5,000 BTUs or less in an air conditioner. Though 100 square feet times 20 BTUs equals 2,000 BTUs, manufacturers consider other factors that affect the cooling capacity of an air conditioner, and recommend higher BTU units to compensate. If your room is oddly shaped, EnergyStar.gov recommends you break the room into geometric shapes, measure the area of each shape and then sum the totals to calculate the area.
Is Bigger Better?
When you select an air conditioner, bigger is not better. In fact, if a unit is oversized for a room, the air conditioner's thermostat will measure a reading cooler than that actual room temperature. As a result, the unit's thermostat will cycle on and off every couple of minutes but will not effectively remove humidity or cool the air. A symptom of a too-powerful air conditioner is a room that feels slightly clammy but not cool and comfortable.
Though you should consider BTUs when choosing an air conditioner, an efficiency rating is also important. The efficiency rating, or EER, describes how much cooling an air conditioner gives per watt of electricity. This is important, especially during the peak of summer, when air conditioning can drive electric bills very high. A high rating frequently translates into greater efficiency and energy savings. According to the American Council for an Energy-Efficient Economy, any new air conditioner you purchase should have an efficiency rating (EER) of at last 10.8.
Other Factors Affect BTUs
There are many factors that should influence which size air conditioner you buy. If the unit is in the kitchen, add 4,000 BTUs to compensate for heat generated from the stove and dishwasher. Add 600 BTUs for each person using a room. And if the room has a southern exposure, increase the BTUs by 10 percent.