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What happens if I choose the wrong size AC? |
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The AC size depends on the size of the room intended to be cooled. Choosing the wrong unit can make an air conditioner inefficient in cooling and removing humidity from the air. The required cooling capacity for a window AC depends on the room's size (including the room’s height), but also on shading and climate or on the sizes of the windows. |
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| Oversizing a Room Air Conditioners |
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Undersizing a Room Air Conditioner |
| A window AC that is too large will not cool the room uniformly. If an air conditioner has a higher BTU rating than needed for the room, the unit will cycle off too quickly and be unable to properly remove the humidity from the air. It is normally preferable to use a smaller unit running for a longer period. The smaller unit will operate more efficiently, and will dehumidify the room more effectively than a larger unit cycling on and off frequently. |
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An Air Conditioner that has a lower BTU rating than the required room size will not adequately cool the room. And will over-dry the air. An average reference – without factoring the climate or shading - to estimate the recommended BTUs is to multiply the square footage by 30 for each square foot of living space. |
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Matching square footage with the recommended BTUs: |
The cooling capacity of room air conditioners is measured in BTUs,
or British Thermal Units, per hour. |
| Area To Be Cooled (square feet) |
Capacity Needed (BTUs per hour) |
| 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 |
Note: This chart is to be used only for the area to be cooled by a single air conditioner. If the room is connected to another by a permanently open door or archway more than 5 feet wide, consider the two rooms as one area and make all the necessary measurements in both rooms. |
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| How do I figure out which AC I need? |
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| Measure square footage: |
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| Calculate the square footage of the room(s) needing the most cooling. Measure the square footage of that room. Then calculate the square footage by multiplying the length of the room by the width of the room. |
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As an example, let's say the room is 18 feet by 25 feet. |
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Length x Width = 18' x 25' = 450 square feet. |
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ENERGY STAR
Earning the ENERGY STAR means a product meets strict energy efficiency guidelines set by the U.S.
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Factors to consider: Since this chart is designed for an average room under average conditions, one should consider any special conditions or needs the room has. |
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- Add 10% BTU if your room is a sunny area, on the second story or in an elevated setting.
- Reduce the BTU by 10% for shaded areas/ rooms.
- Add 600 BTUs for each person if the room is occupied regularly by more than two people.
- Add 4,000 BTUs if the AC is used to cool a kitchen.
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U.S. Department of Energy
Information for Consumers About Air Conditioning. |
If ceilings are higher than 8 feet, or the room is located directly under the attic, cooling needs might also indicate an AC BTU classe higher than that suggested by the chart. |
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| Determine the square footage of the area to be cooled using the following formulas: |
| For square and rectangular rooms, multiply the length of the area by its width For a triangular area, multiply the length of the area by the width and divide by 2. Most rooms can be further divided into these basic shapes to determine the square footage. |
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| Additional Recommendations: Energy Efficiency of Room Air Conditioners |
| The efficiency of a room AC is measured by its EER (Energy Efficiency Ratio) rating. The EER is the relation between the cooling capacity and the watts power input. Replacing an old room AC with a EER of 6 with a 12 EER one, will reduce air conditioning by half. Purchase a window air conditioner with low EER ratings. Buy equipments with an EER of at least 12. |
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