How to select the right size AC ?

How to calculate the perfect size ac for a room?

While buying new air conditioner, we always have question in our mind that what is the perfect size ac we need to suite our requirement? Well calculating the ac capacity is not difficult. You can apply rule of thumb.

Ac’s cooling capacity is measured in TON or Tonnage, Btu/h (British thermal unit) or in kW (Kilowatt). In country like USA and India it is measured in tonnage (Also measured in btu/h and kW as well).

1 ton refrigeration = 12000 btu/hr = 3.517 kW

Conversion
Btu/h	Refrigeration Ton	kW
6000	1/2	1.76
12000	1	3.52
18000	1 1/2	5.28
24000	2	7.03
30000	2 1/2	8.79
36000	3	10.6
42000	3 1/2	12.3
48000	4	14.1
54000	4 1/2	15.8
60000	5	17.6

We will consider following room size for our calculation: Width 20ft x Length 20ft x height 10ft

Calculation of AC (Air conditioner) size (Capacity)

Rule of Thumb 1 (By calculating Area): Multiply the length and breadth of the room in feet. Divide the square root of the answer by 10. Final answer will give the desire cooling capacity of ac in tonnage.

Example 1:

1. We will calculate ac size (Cooling capacity) for room which is 20ft wide x 20ft long.
2. Multiplication of length and breadth is 20 x 20 = 400 square feet
3. Square root of 400 is 20
4. Divide the Square root by 10 is 20/10 = 2
5. AC cooling capacity should be 2 ton.       

Rule of Thumb 2 (By calculating Volume): Multiply the height, length and breadth of the room in feet. Divide the cube root of answer by 10. Final answer will give the desire cooling capacity of ac in tonnage.

Example 2:

We will calculate ac size (Cooling capacity) for room which is 20ft wide x 20ft long x 10ft high

1. Multiplication of height, length and breadth is 20 x 20 x 10 = 4000 cubic feet
2. Cube root of 4000 is 15.87
3. Divide the answer (15.87) by 10 is 15.87/10 = 1.58
4. AC cooling capacity is 1.58 ton.

Followings are some other factors which determine the AC (Air conditioner) capacity.

Sunlight: If your room is exposed to direct sunlight during the day you need to increase required capacity by 10%.

Number of occupants: Above basic calculation is considering minimum 2 occupants per room. You should add additional 0.05 tonnage (600 btu/hr) cooling capacity for each person.

Kitchen: Increase another 0.33 tonnage (4000 btu/hr) if you are installing ac for kitchen.

Let’s calculate cooling capacity again considering above mentioned points

Again, we will take same example. We will consider our room size is Width 20ft x Length 20ft x Height 10ft

1. As we calculated in the example 2 required cooling capacity is 1.58 tonnage. If room is directly exposed to the sunlight, we will add 10% means 10% of 1.58 = 0.158 tonnage.
2. If total number of people in the room is 4, then for additional 2 persons we need to add, 2 x 0.05= 0.1 tonnage
3. It this ac is for kitchen then another 0.33 tonnage need to add.
4. Final addition is if this ac is not for kitchen, 1.58 (Required capacity) + 0.158 (direct sunlight) + 0.5 (additional 2 persons) = 1.838 tonnage
5. If this ac is for kitchen another 0.33 tonnage need to add so for kitchen the required capacity is = 1.58 (Required capacity) + 0.158 (direct sunlight) + 0.5 (additional 2 persons) + 0.33 (For kitchen) = 2.168 tonnage

What if we buy under (Smaller) capacity AC (air conditioner)?

Smaller capacity unit will run continuously to achieve the required temperature which result in excess consumption of electricity, means will increase the electricity bill.

What if we buy over (Bigger) capacity AC (air conditioner)?

Bigger capacity air conditioner will cool room quickly. But if your ac does not run long it will not extract moisture from the air as it takes some time. This quick cooling will cause sticky and hot feeling. Ac also will turn on and off frequently which is not good for compressor in long run.

Pro tips

Few other factors need to consider, like how many appliances like refrigerator, television, fans, electric tube light or Bulbs are there in the room. Infiltration through door and window and number of door openings also play a major role. Split ac is always better than window ac as it is mounted min 6 to 7 feet above the floor height. Cold air from higher point has its advantage to cooling the room thoroughly as cold air is heavier than hot or warm air. Mounting and removing of indoor unit is easier than the window ac.