An air conditioner that is too small runs all day and never reaches the set temperature. One that is too big cools the room in short bursts, switches off before it has removed the humidity and leaves the air cold and clammy, while wearing out faster. The right size depends mainly on the floor area, then on the sun, the number of people and heat from cooking.
The Energy Star sizing chart
| Room area (sq ft) | Cooling capacity (BTU/h) |
|---|---|
| 100 to 150 | 5,000 |
| 150 to 250 | 6,000 |
| 250 to 300 | 7,000 |
| 300 to 350 | 8,000 |
| 350 to 400 | 9,000 |
| 400 to 450 | 10,000 |
| 450 to 550 | 12,000 |
| 550 to 700 | 14,000 |
| 700 to 1,000 | 18,000 |
| 1,000 to 1,200 | 21,000 |
| 1,200 to 1,400 | 23,000 |
| 1,400 to 1,500 | 24,000 |
| 1,500 to 2,000 | 30,000 |
| 2,000 to 2,500 | 34,000 |
The chart assumes 8 ft ceilings and ordinary insulation, about 20 BTU/h per square foot. Above 2,500 sq ft, the calculator continues at 14 BTU/h per square foot, but a whole-house system should be sized by a contractor with a Manual J load calculation.
Adjustments
- Heavily shaded room: reduce capacity by 10%.
- Very sunny room: increase by 10%.
- More than two regular occupants: add 600 BTU/h for each extra person.
- Kitchen: add 4,000 BTU/h for the heat of cooking.
Example
A 400 sq ft living room with normal sun and two people needs about 10,000 BTU/h, or 2.9 kW of cooling. The same room facing west with large windows needs about 11,000 BTU/h, so a 12,000 BTU unit would be the next standard size.
Units: BTU, tons and kilowatts
One ton of refrigeration is 12,000 BTU/h, the heat needed to melt a ton of ice in a day. One kilowatt of cooling equals 3,412 BTU/h. Window and portable units are sold in BTU; central systems and mini-splits are often rated in tons.
Choosing a type
- Window units are cheapest for a single room.
- Portable units are easy to move but less efficient; single-hose models pull warm outside air into the room. Their capacity under the newer SACC rating is lower than the old ASHRAE figure, so compare SACC numbers.
- Ductless mini-splits are quiet and efficient, and also heat as heat pumps.
Look at the efficiency rating (CEER for room units, SEER2 for central systems): a higher number means lower electricity bills.