How to Size a Room Air Conditioner

To buy an energy-efficient room air conditioner, you must first determine what capacity, or size, you need. This is important for two reasons:

• An undersized unit won't cool adequately in extremely hot weather.
• An oversized air conditioner can also adversely affect your comfort. The unit may switch on and off too often, without running long enough to dehumidify the room properly or cool the space uniformly. And it will consume more energy than necessary.

Use the simple calculation method described below to estimate the cooling capacity you'll need to maintain a temperature of 24°C with a humidity level of between 30 and 50 percent. You can find a more detailed calculation method in another NRCan publication entitled Air Conditioning Your Home (the "Useful Information Sources" page explains how you can order this booklet or view it on-line). If you're cooling a large space, consult a qualified contractor who will consider such factors as the amount of space to be cooled, the number of occupants, the insulation levels and the size, orientation and R-value of windows and doors.

Step 1

Calculate the floor area of the space to be cooled.

To calculate floor area, multiply the length of each room or open space by its width. Don't estimate - take a few minutes to measure accurately. Units are often bought to cool one or two rooms. Enter your numbers into the blanks provided. (The calculation for a typical open-concept living/dining room will guide you through the process.)