air conditioner sizing

Most air conditioners are oversized by 50%, 100%, or more, according to several official accounts. And most people still think that oversizing is a good practice, providing better cooling. But this is nonsense.

Oversized air conditioners

An oversized air conditioner is less effective and wastes large amounts of energy. If the air conditioner unit is oversized, it will cool the room quickly but it will also remove some of the necessary humidity, leaving the room with a damp feeling. Only a properly sized unit removes humidity effectively as it cools.

A smaller unit is also less environmentally-unfriendly, quieter and allows higher comfort (oversized air conditioners will cycle on and off more frequently, making indoor temperatures fluctuate more). Besides, a smaller non-oversized unit is also more appropriate to most ducts, and will last longer.

Make sure you buy an air conditioner with the right size (a too small unit is also a wrong decision: an undersized will over-dry the air and will not cool it well).

Before buying

Before buying an air conditioner, be sure to air seal and to improve your home's insulation and shading.

You should implement shade on your home's windows, roof and walls (through reflective roof coatings, awnings, or trees and landscaping). And you should insulate your home's attic, ceiling and walls, and fix possible air leaks…

With these measures you will get lower air conditioning needs and you will need a smaller air conditioner unit.

BTU and tons

The size of an air conditioner is commonly measured by the number of British Thermal Units (BTU) of heat they can remove (per hour). The ton (12,000 Btu per hour) is another common rating term, but BTU is more universal.

Sizing and square footage

The size of an air conditioner depends largely on the square footage of the house. The chart below – suggested by Energy Star - can help you find the appropriate power for a (room) AC…

Note: Most room AC have cooling capacities between 5,500 Btu and 14,000 Btu per hour. 12,000 Btu per hour is often said to be the equivalent to a «ton».

Area To Be Cooled (square feet)	Area to Be Cooled (m2)	Capacity Needed (BTUs per hour)
100 to 150	9.3 to 14	5.000
150 to 250	14 to 23	6.000
250 to 300	23 to 28	7.000
300 to 350	28 to 33	8.000
350 to 400	33 to 37	9.000
400 to 450	37 to 42	10.000
450 to 550	42 to 51	12.000
550 to 700	51 to 65	14.000
700 to 1,000	65 to 93	18.000
1,000 to 1,200	93 to 111	21.000
1,200 to 1,400	111 to 130	23.000
1,400 to 1,500	130 to 139	24.000
1,500 to 2,000	139 to 186	30.000
2,000 to 2,500	186 to 232	34.000

Adjustments

Energy star also advises to…

- reduce the needed capacity by 10% if the room is very shaded, or increase it by 10% in the opposite case;
- increase the capacity by 600 BTUs per person, if the room is occupied regularly by more than two people;
- increase the capacity by 4,000 BTUs in the case of kitchens.

If ceilings are higher than 8 feet, or if the room is located directly under the attic, then your cooling needs might also be one to two Btu classes higher than that suggested by the Energy Star chart.