BTUs and EER

• Window air conditioning units range from 5,000 to 30,000 BTUs. The efficiency of air conditioners is measured by the energy efficiency ratio, the EER. The EER is the ratio of the cooling capacity in BTUs and the power input in watts. Look for air conditioners with an EER of 10.0 or above. A more energy efficient window air conditioning unit has a higher EER number.
  
  

Quick Method to Estimate BTU Requirement

• To quickly estimate the required size of a window air conditioner, measure the length and width of the room. Multiply the length by the width by 35. This gives an estimate of the BTU requirement. For example, if a room measures 12 feet by 16 feet, multiply 12 by 16 by 35; this room requires an air conditioner with about 6,720 BTUs. If the room is sunny, increase the BTUs by 10 percent. Reduce the BTUs by 10 percent for a shady room. If the air conditioner is mounted in a kitchen window, add 4,000 BTUs. If two or more people will constantly occupy the room, add 600 BTUs per person.
  
  

Longer Method to Estimate the BTU Requirement

• Measure the room height, width and length. Determine the orientation factor by finding the wall with the most window area. If the wall faces north, the orientation factor is 16. The factor for an east-facing wall is 17. A south-facing wall is 18, and a west-facing wall is 20. Estimate the insulation factor. A house with a well-insulated roof or attic is given a rating of 10. A house with uninsulated ceilings or roof has a rating of 18.
  
  

Formula for BTU Requirement

• Multiply the room height by the room width, then by the room length, the exposure factor and the insulation factor, and then divide by 60. A 12-foot by 14-foot room with an 8-foot ceiling, a well-insulated roof and an east-facing orientation has a 3,808 BTU requirement. This is calculated by multiplying 12 by 14 by 8 by 10 by 17, and dividing by 60.