All of your air conditioning units operate at a certain efficiency. And, of course, the higher the efficiency the less it costs you to operate them. Current models generally are 30% more efficient than those of a decade ago and now carry up to 10-year warranties.

The efficiency of central air conditioners is expressed in terms of Seasonal Energy Efficiency Ratio (SEER). The higher the SEER, the higher the efficiency. Units with a SEER over 13 are considered high efficiency. Currently, the highest SEER available is 21.

Heat pump cooling efficiency is measured by a SEER type of rating called Heating Seasonal Performance Factor (HSPF). If you have a unit with an HSPF of 8 or more and a SEER of at least 12, then you have what is considered a high-efficiency unit.

The efficiency of room air conditioners is measured in terms of Energy Efficiency Ratio (EER), which is determined by how much heat the unit is removing compared to how much electricity it is using. The higher the EER, the less electricity the unit will use.

The capacity of room air conditioners is established by their capacity to remove heat, which is expressed as British Thermal Units per Hour (BTUH). This can range from 5,000 to 25,000 BTUH.

Many factors come into play when sizing your air conditioners – such as size and type of windows and which directions they face, wall construction, insulation, roof type, etc. Your local licensed professional contractor takes all of this into account when making recommendations.

If your central or room air conditioners are too small, they will not cool adequately. But if they are too large, they will cool so quickly that their thermostats will shut them down before they can lower the humidity enough, and then your space will feel damp and clammy.

Cooling a 100- to 250-sq-ft space might require 5,000 to 6,000 BTUH while cooling 875 to 1,200 sq ft might require 15,000 to 19,000 BTUH, depending on the many factors that affect the heat levels in your house or apartment. So get advice from your local licensed professional contractor.