How to Estimate the Capacity of Air Conditioners

An oversize air conditioner can be just as inefficient as one that is too small.

An air conditioner's cooling capacity is measured in BTU/h (British Thermal Units per hour). An air conditioner that is too small will not give you sufficient cooling. An oversize unit will not have time to remove humidity from the room, which is an important factor in air cooling. If you have an air conditioner but do not know the cooling capacity, this can be found in the user's manual, or on the unit itself. Estimate the capacity of an air conditioner suitable for your home or office in a few steps.

Instructions

1. Measure the length and width of the rooms to be cooled, and write those measurements down.

2. Calculate the floor area of each room by multiplying the length and width. For example, 20 feet times 50 feet is a room of 1,000 square feet. If you have more than one room to be cooled with the air conditioner, add the floor area of each room to arrive at the total floor area.

3. Estimate the BTU/h required based on the following preliminary numbers recommended by the National Research Council of Canada. They are based on a space normally occupied by two people. For room space from 100 to 150 square feet, use 5,000 BTU/h. For spaces from 140 to 250 square feet, use 6,000 BTU/h. For 250 to 300 square feet, use 6,500 BTU/h. For 300 to 350 square feet, use 7,250 BTU/h. For 350 to 400 square feet, use 8,000 BTU/h. For 400 to 450 square feet, use 8,750 BTU/h. For 450 to 500 square feet, use 9,650 BTU/h. For 500 to 550 square feet, use 10,500 BTU/h. For 550 to 700 square feet, use 12,500 BTU/h. For 700 to 1,000 square feet, use 15,000 BTU/h. For 1,000 to 1,200 square feet, use 17,700 BTU/h. For areas between 1,200 to 1,600 square feet, use 19,000 to 24,000 BTU/h. For areas up to 1,800 square feet, use 24,000 to 27,000 BTU/h. For larger areas up to 2,800 square feet, use 33,000 BTU/h. Write down this number.

4. Subtract 10 percent from the number in Step 3 if the room faces north or northeast, or is heavily shaded.

5. Subtract 30 percent from the number in Step 3 if the air conditioner will only be used at night.

6. Add 10 percent to the number in Step 3 if the room is extremely sunny.

7. Add 15 percent to the number in Step 3 if the room is not well-insulated.

8. Add 4,000 BTU/h to your number from Step 3 if the space being cooled includes a kitchen.

9. Add an additional 600 BTU/h for each extra person over two normally occupying the space to be cooled.

References

• National Research Council: How to Determine the Right Size of Air Conditioner
• Inspectapedia: How to Determine Cooling Capacity of Air Conditioning Equipment