When you are looking for a window air conditioner or a portable A/C unit, it is easy to determine the correct size using some simple math. Choosing the correct air conditioner size assures that the unit will achieve a comfortable level of cooling and will not strain a budget any more than is necessary.

Air conditioning units are rated in BTUs (British Thermal Units). A BTU is actually a measure of heat, but BTUs are used for the rating based upon the amount of heat that the air conditioning unit can remove per hour. The simple physics of cooling states that an object can be warmed by applying heat, but in order to cool an object, heat must be removed. An air conditioner’s capacity is therefore rated based upon how much heat it can remove from a room.

To calculate the needed BTU rating, simply calculate the square footage of a room and multiply that number by 35. In other words, if you wish to purchase a window air conditioner and the room you need to cool is 10 feet wide by 20 feet long the correct size for A/C unit is:

**15 x 20 = 200 square feet, 200 x 35 = 7,000 BTU air conditioner.**

When calculating the size for a window air conditioner in a south-facing window, add 10% to the calculation.

Determining the size for an air conditioning system for an entire home is more complex, because the estimator has to take into account amount of insulation in the walls and attic of the home, as well as other factors that can affect the cooling ability.

Also, check the efficiency ratings for the the AC units being considered. The higher the efficiency rating, the less it will cost to run the unit.

Other factors to consider include the available power to run the unit. Some larger window air conditioners require a dedicated circuit for safety. Air conditioners do draw a relatively large amount of power, which means it cannot be plugged into a circuit that already has appliances or other electrical devices that also draw a lot of power.