Air conditioners with 6,000 British Thermal Units (Btu) of power will cool a 150- to 250-square foot room. The air conditioner draws in the heat and humidity. As the air flows over the refrigerant coils inside, the humidity is removed and the air is cooled before returning back to the room. The air conditioner's effectiveness within a room can increase or decrease depending on shade, sun and how many people are in the room.
Measuring the Room
Room sizes are determined by measuring the room's width and length, which are multiplied to find the room's square footage. If the room size is between 150 to 250 square feet, a 6,000 Btu air conditioner will cool the room efficiently. Rooms that are less than 150 square feet or more than 250 square feet shouldn't employ a 6,000 Btu window unit.
Sizing the Unit
Running a 6,000 Btu window unit in rooms larger than 250 square feet will result in the air conditioner running all the time and not cooling the room completely, but leaving the room muggy. In a room smaller than 150 square feet, the same unit will cool the air quickly, but won't remove the humidity, which will cause the room to feel damp. The unit will continually shut off and start up, wasting more energy.
If more than two people will be in the room being cooled, an air conditioner's effectiveness is reduced by 600 Btu for each additional person. For instance, if three people are in a room, a 6,000 Btu unit will cool 100 to 150 square feet effectively.
Shade or Sun
Rooms that are heavily shaded from the sun will increase the unit's effectiveness by 10 percent. For instance, if the 6,000 Btu unit is in a shaded room, the air conditioner can cool 165 to 275 square feet effectively. Rooms that receive direct sunlight will decrease the same unit's effectiveness by 10 percent, down to 135 to 225 square feet.