To determine how much area an window air conditioner will cover, you need to know several factors about the room being cooled. These are the room size in cubic or square feet, the size of the window you will put it in, the capacity and energy efficiency of the air conditioner and the amount of direct sunlight that will shine on the unit. Also assess your home's insulation effectiveness. Use these tips to work out the exact size of air conditioner you need for effective cooling.
Measure the room's size in cubic feet (length times width times the room's height), not just the square footage. This will tell you the room's air flow and volume that will be cooled.
Measure the window carefully where you intend to install the air conditioner. Use a custom-cut insulated foam panel to seal a window larger than the air conditioner. The more airtight the seal, the better your window air conditioner will work.
Air Conditioner Capacity and Energy Efficiency
Air conditioners measure their cooling capacity in BTUs, or British thermal units. Air conditioners range from a small 5,000-BTU unit to those that can cool an entire house, at 24,000 BTU. Check the energy efficiency rate of the air conditioner you want to buy. An air conditioner with an EE ranking of 10 will cool twice as well as an EER5 unit. The minimum EER for air conditioners made and sold today is 8.
Direct Sunlight Level
You also need to know the direct sunlight level on the window air conditioner wall. The four compass directions have these direct sunlight values: north is 16, east 17, south 18 and west 20. In the calculation below, choose the appropriate value and multiply it by the cubic footage (air volume) of the room being cooled.
Your Home's Insulation
The quality of your home's insulation will also affect how well your air conditioner cools the room. If your insulation is excellent, use a value of 6 for the formula. If it is of medium effectiveness, put a value of 5 into the formula. If it is poor with plenty of drafty windows and doorways, assign it a value of 4.
Calculate the Air Conditioner Capacity You Need
The cooling capacity formula is Cubic Footage of Room times the Direct Sunlight Level, divided by the Home Insulation Value. A living room that measures 15 feet by 18 feet, with a 9-foot ceiling has an air volume of 2,430 cubic feet. If the windows face east, its direct sunlight level is 17. Multiply 17 x 2,430 = 41,310. Divide this number by your insulation value:
For a well-insulated home, 41,310 / 6 = 6,885. A 7,000 BTU air conditioner will work fine.
For medium insulation, 41,310 / 5 = 8,262, so get an 8,500 BTU unit.
For the lowest insulation, 41,310 / 4 = 10,328. You will need a 10,500 BTU unit.
Never buy an air conditioner well above the capacity you need, as it will waste energy in turning on and off frequently.