How do you know what strength of air-conditioning unit to buy?

The most important factor in choosing an air conditioner is the intended use. You need to know the total square feet of your home so that you buy one that is going to be able to cool the entire area efficiently. This is called a load calculation. Also, you may qualify for a federal tax credit if you buy the proper SEER Energy Star air conditioner or heat pump. (see related link)

The "strength" of an air conditioner is rated for BTUs (British Thermal Units), the unit's total cooling capacity, as well as for the volume of air circulated (in tons of air).

Generally, smaller self-contained units or window-mounted units will have their cooling power noted in BTUs, and several thousand is the norm. Commercially-available units will specify an approximate effective square-footage or room volume. Too small, and the unit will be unable to cool the space properly, and too large is simply overkill and a waste of electricity (and extra cost for the unit).

Larger, residential "central air conditioning" have massive cooling capacities, and quoting their capacity in BTUs becomes unwieldy. Instead, the tonnage of the unit (mass of air moved) is typically quoted in the industry. 3.0, 3.5, and 4 ton units are typical for most average-sized homes ranging from 1,500-2,500 square feet. High ceilings, glass doors and Windows, and construction of home can make a difference as well. If you have dual pane Windows and insulation, your needs may be lower.

Like in the window-unit scenario, bigger isn't always better; overkill is expensive to buy and maintain. A larger unit may also run for a shorter time, and sometimes not draw out enough moisture from the air (making it feel less comfortable) than a smaller unit running for longer. These differences are not always intuitive, so a professional evaluation is generally desirable.