How To Find The Proper Aircon Capacity For Your Room Size

Choosing the right aircon capacity for your room size will have a big effect on your comfort and your electricity bill. If you choose a unit that is too small, it won’t cool your room quickly and efficiently; too big, then it will cool too quickly without removing the humidity – making you cold and clammy.

When referring to an air conditioner’s size, we are not talking about its physical dimensions. Instead, we are referring to its cooling capacity. In this article, I will give you a quick method to help you find the proper aircon capacity for your room size.

How an aircon’s capacity is measured

An air conditioner’s cooling capacity is measured in either kilojoules/hour (kJ/hr) or British Thermal Units (BTU). This is used to measure the amount of heat your aircon can remove from the air per hour of use. In the Philippines, we mostly use kJ/hr so you should use this for your calculations.

Aside from kJ/hr and BTU, an aircon’s capacity can also be measured in horsepower (HP). However, HP is used to measure the motor’s power, not the aircon’s cooling capacity. A unit with the same horsepower rating may differ in their cooling capacity. Therefore it is much more accurate to use kJ/hr than HP when calculating the capacity of your aircon. It’s not totally useless though, as you can use it to filter the models that you will canvass. Keep in mind that 1.0HP = 9,495 kJ/hr.

Aircon capacity to room size guide

An air conditioner’s recommended floor area is measured in square meters (sqm or m2). If you have your house’s floor plan, this will be easy. Otherwise, grab a tape measure and measure the room’s length and width and multiply both numbers to get the room’s size in square meters.

To get a rough calculation, multiply your room size by 500. So if your room is 10 sqm. in size, you need an aircon with 5,000 kJ/hr of cooling capacity – which most 0.5 HP window type aircons have. Below is a cheat sheet of the base capacity per room size. To get a more accurate calculation, there are other factors that you need to account for, which we will discuss in the next section.

Room Size Capacity (kJ/hr) Estimated HP Rating
≤ 10 sqm. 5,000 0.5 HP
10 - 15 sqm. 5,000 - 7,500 0.75 HP
15 - 19 sqm. 7,500 - 9,500 1.0 HP
19 - 25 sqm. 9,500 - 12,500 1.5 HP
25 - 35 sqm. 12,500 - 17,500 2.0 HP
35 - 43 sqm. 17,500 - 21,500 2.5 HP

Other factors to include

Knowing your room’s size will give you a nice starting point to estimate the size of your aircon. However, to get a more accurate estimation, you should take into account your room’s heat load. The heat load (or heat gain) refers to the sum of the heat (expressed in kJ/hr) emitted by heat sources inside a room.

The room size is one factor. Other factors include the number of occupants, windows, and appliances inside the room. So, take the baseline cooling capacity based on its size, then adjust your calculations depending on the following circumstances:

1. Shade

The sun is one of the biggest factors that determines the heat load. If your room is subjected to direct sunlight, you should add 10% to the room’s cooling capacity; if it is shaded, then deduct 10%.

2. Number of occupants

Humans also emit body heat that needs to be accounted for. If the room is used by two people, leave the calculation as is. However, if it is used by more than two people, then add 633 kJ/hr for each additional person.

3. Lighting and other sources of heat

Light bulbs are also sources of heat when they are in use. To get a simple calculation of their heat loads, look for their rated wattage and use the following calculation for each lamp:

((Rated Wattage * Hours of Use) ÷ 1,000) * 3,600

The calculation is divided by 1,000 to convert the watts per hour into kilowatts per hour. The product is then multiplied by 3,600 to convert the kilowatts per hour to kilojoules per hour. Add all of the heat loads of each appliance then add the sum to your baseline cooling capacity.

Another thing to add, if you have a stove in your room (especially for studio type condo dwellers) add an additional 4,220 kJ/hr to the calculation.

Sample calculation

Here is a sample heat load calculation to see the formula in action:

Given: your room is 20 square meters in size with a lot of windows that lets the sunlight in and with 3 constant occupants. The room is illuminated by one LED ring light (15W). Calculate the required cooling capacity and get the recommended model size in HP.

How to calculate: first up, you’ve have to calculate the heat load of the LED ring light. Let’s assume that the light is turned on for only 8 hours.

Item Wattage Hours of Use In kWh (divided by 1000) In kJh (multiplied by 3600)
LED Light 15 8 0.12 432

After that, you should get the baseline capacity for the room size and add 10% to the baseline because the room is sunny (represented here as “* 1.10” to simplify the calculation). Next, add the other factors like the heat load from the additional occupant and the lighting. Your calculation should look like this:

Calculation:

Step 1: (Room Size x 500) * 1.10 + additional occupant + lighting
Step 2: (20 x 500) * 1.10 + 633

Step 3: 10,000 * 1.10 + 633
Step 4: 11,000 + 633

Required cooling capacity: 11,633 kJ/h
Recommended model size: 1.5 HP

Conclusion

Bigger is not always better. If the aircon is too large, it will use up more energy, take away less humidity, and will have a higher wear and tear compared to the proper-sized models. Smaller is not ideal as well. A small air conditioner will struggle to cool bigger rooms, which will use up more energy, and will strain your unit’s motor.

With this method of calculation, you can gauge accurately where the sweet spot is. If you’re not confident with finding this Goldilocks zone on your own, don’t hesitate to call a professional! Also, if you’re still not sure what kind of air conditioner to buy, you should read this aircon buying guide to get you started.

6 thoughts on “How To Find The Proper Aircon Capacity For Your Room Size”

  1. Identifying the proper aircon capacity for your room help you to choose the bet air conditioner to provide better cooling for you room. The information that you have shared it with us will help us in doing that better. Thank you for sharing it with us.

    Reply
  2. This is a great article, a rare information-packed and region-based, indeed! Most articles I’ve read were less details and mostly paraphrased from just one source. Nowadays, you can really tell if an author is just basing purely on concepts or with fact combined with priceless experience.

    I came across this site after googling “how to calculate hp for aircon” and since it only ranked 3rd, I’ve read others’ articles first. But this is the only article which considers more parameters in choosing the right ACU, especially here in the Philippines where I am living.

    But, I still have one more parameter in mind that’s part of room size that most articles I’ve read haven’t fully accounted for while the rest are totally silent about it. This is the height of the room.

    I’m really wondering which room will be easier to cool, a 20sqm room with a standard height of 2.7m or about 9feet or a room with the same area but 3.7m or about 12 feet height?

    I understand that hot air rises and cold air sinks that’s why most well-ventilated rooms have 12ft or higher floor to ceiling ratio. But what is the total effect height with ACUs?

    I hope to read “Mores” from you, Miguel! Always keep safe and thanks for this well-written article. 🙂

    Reply
  3. Great information here. An AC website calculator suggested to get a higher HP but the additional cost is too much for us. Good thing i found your site. I think the lower HP would be enough for our room. Thank you.

    Reply
  4. Thank you so much for this highly informative article. Excellently presented for us ordinary consumers! More power to you!

    Reply

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