Room air conditioners are generally sized in Btu (this actually means Btu per hour). One ton is equivalent to approximately 12,000 Btu per hour. Rough guidelines for sizing a unit are presented below. Energy efficiency of room air conditioners is measured as an Energy Efficiency Rating (EER). New room air conditioners must have a minimum EER of about 8.6, depending on the size of the unit.

Rough Sizing of Room Air Conditioners

Maintenance
Regular maintenance of a room air conditioner not only improves efficiency but also extends the useful life of the unit. As warm weather approaches, the two things to check are the air filter and the cooling fins.

Air Filter - Replace or clean if it is dirty and continue to check it every month. Replace or clean as necessary. Cooling Fins - Found on the back and front of the air conditioner, the cooling fins are a series of thin metal sheets with a very small gap between them. The cooling fins should be evenly spaced, but often they get damaged and rub up against one another. To separate the fins a fairly inexpensive but essential tool is a fin comb, available at most refrigeration parts stores and some building supply stores.

If you notice the performance dropping off and the filter and fins are okay, then take the unit to a repair shop for service.