Choosing the right aircon capacity for your room size will have a big effect on your comfort and your electricity bill. If you choose an AC unit that is too small, it won’t cool your room quickly and efficiently. However, if it is too big, then it will cool your room too quickly without removing the humidity. This makes your room uncomfortably 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 when referring to an aircon’s cooling capacity so you should use this in 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. That is to say. different models with the same horsepower rating may differ in their cooling capacity. Therefore it is much more accurate to measure models by kilojoules per hour than by horsepower rating when calculating the capacity of your aircon.
The horsepower rating is not useless though, as you can use it to filter the models that you will be considering. 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 air conditioners 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:
The amount of sunlight in a room is one of the biggest factors that determine its heat load. If your room is subjected to direct sunlight, you should add 10% to the room’s cooling requirement; if it is shaded, then deduct 10%. It’s a big reason why you should think about installing proper shades in your room to save energy.
2. Number of occupants
Humans also emit body heat. Hence it needs to be accounted for to get an accurate measurement of the room’s heat load.
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 those living in studio type rooms) add 4,220 kJ/hr to the 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 let 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 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)|
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:
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
Bigger is not always better. If the aircon is too large, it will use up more energy, take away less humidity, and will have 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.
Miguel Mores worked for 5 years as a member of the product management team for a home appliance company in the Philippines. He started 101appliance to answer the most common customer questions that he has encountered during his time in the industry. He now works in the digital marketing field and manages a small online bookstore on the side.