Summers bring the dread of heat. High temperature causes discomfort, sweating, and faster exhaustion. We’ve used fans to have the air flowing, but as the summer advances, air conditioners become necessary to provide a comfortable environment. Modern air conditioners provide fast and effective cooling. Recent advancement in technology has made aircons more energy-efficient too. The question that pops up when considering an aircon purchase is – How much capacity do I need?
Let’s first understand air conditioning capacity
An aircon’s cooling capacity is measured in tons. This capacity is often referred to as the “tonnage” of the air conditioner. Simply put, higher the tonnage, greater is the aircon’s capacity to cool. Said another way, an AC with a higher tonnage can cool a greater area.
If you’re considering an air conditioner for your room, its capacity would primarily depend on the area of the room and how much heat enters the room externally. The AC is essentially a heat pump that removes heat from your room. Tonnage refers to the rate at which an aircon can remove heat. Higher the tonnage, faster is the heat removal.
Capacity may be represented in various forms. The essence remains the same – “How much heat can the aircon remove per second.”
How is AC capacity measured?
Ton (tonnage), BTU per hour, and Watts are the three most common measures of air conditioning capacity. Since all the three measure the same aspect, there must be a way to compare and connect them.
Let’s be very clear – tonnage has no connection whatsoever to the weight of the air conditioner.
1 Ton of air conditioning = 12,000 BTU per hour = 3500 Watts of cooling (approx.)
You may have seen watts in the power consumption section of the air conditioner. That number measures how much energy the system consumes every second, whereas capacity indicates how much heat the system removes every second. BTU stands for British Thermal Unit and is a standard measure of heat used decades back. For your curiosity, it is the amount of heat released when a match stick is burnt all the way.
Now that you know the equivalence, it is simple mathematics.
2 Tons of air conditioning = 24,000 BTU per hour = 5150 Watts cooling (approx.)
At least one of these three numbers are mentioned on the box or specifications sheet. It would allow you to make an informed decision on the capacity of air conditioning you need.
Factors affecting AC capacity requirement
Climactic conditions are the primary determiners of AC capacity. If you live in a tropical area, the high temperature and humidity would require a higher capacity aircon as compared to a place which has relatively lower temperature and humidity.
Here is a detailed list of the factors affecting cooling requirements.
- Room area (square feet is the commonly used measure)
- Climate – maximum temperature and humidity
- Number of individuals typically occupying the room during AC usage
- Ceiling height
- Presence of kitchen
- Heat generated by other electrical appliances
- Total area of windows in the room
- Floor number and building height
- Are your premises located on the top floor?
- Construction material used for walls, doors, and windows
- Points of air leaks in the room
We’ll now see how to calculate the air conditioner’s capacity using the abovementioned parameters.
Calculate the area of the room for which the aircon is being sized. Once you have the area in square feet, follow the indicative capacity, as mentioned below.
Room size (square feet)
Indicative capacity (Tons)
Indicative Capacity (BTU / hr)
150 to 250
250 to 400
400 to 550
This table is meant just for reference. Other factors significantly affect these calculations. The calculations in the table assume that two individuals occupy the room during aircon operation.
If the temperature crosses 45 degrees Celsius frequently, the size requirement increases significantly. Add 6000 BTU per hour to the number you have calculated until the previous step if the climate in your area has many days where the temperature would be over 40 degrees. Since heat flows from a higher to lower temperature, the compressor needs to be powerful enough to discharge excess heat even when the ambient temperature is very high.
The human body releases a lot of heat. An individual at rest would produce 100 W of heat. Depending on the activity level, this number may rise significantly. If you are choosing an aircon for your home gym, its capacity requirement would be significantly higher than one for a bedroom because of the calories burnt during exercise. For every additional occupant in the room, add 600 BTU per hour as additional cooling capacity.
Taking an example, a 450 sq. feet hall would require a capacity of 12000 BTU per hour indicated by the table. However, considering that ten persons may occupy the hall in case of a party or gathering, an addition of 600 BTU per hour per person would effectively increase the capacity requirement by 50%. Given the fact that occupancy is only one factor, the capacity should be chosen sensibly to avoid an undersized or an oversized aircon.
If the height of the ceiling is significantly higher than the 10-12 feet, add additional capacity in proportion. If your ceiling is 2 feet higher than average, it demands a 15-20% increase in air conditioning capacity since the aircon cools the entire volume of air in the room.
Add about 2000 BTU per hour of extra capacity in case of a higher-than-normal ceiling.
Presence of Kitchen
Cooking requires heat. Heat requires more air conditioning; if you’re sizing the air conditioner for a hall or any other area that has a working kitchen add 4000 BTU per hour to the required capacity.
Cooking and air-conditioning are tasks that may not be done simultaneously since they oppose each other. If your room is heavily air-conditioned, the microwave and the induction heaters consume more energy to cook the food.
Conversely, this added heat would prompt your air conditioner compressor to run longer to remove the excess heat.
Heat generated by other electrical appliances
While it may seem little, it is the small bits that add together and form a sizeable chunk. This applies to the small electronic gadgets in the room, as well as any large appliance that you may be using.
A 50-inch LED TV would consume between 50-100 watts of power, most of which would be dissipated as heat.
LED tube lights and bulbs consume between 10 watts to as much as 100 watts. An efficient ceiling fan would consume 50 watts, while a laptop would typically dissipate between 10 watts and 50 watts power.
This case is commonly applicable to home-offices, where multiple employees use laptops, mobile phones, laser printers, etc. Add applicable BTU per hour to the capacity as per the applicable conditions.
The total area of windows in your room
Bigger windows would let the ambient heat “steal” your room’s cooling. If the windows are facing towards the North or Northeast, subtract about 10% of the basic capacity decided in the first step.
On the other hand, if the windows face towards the west or southwest, add about 10% of the basic capacity determined initially.
Total number of floors (in case of a high-rise)
Heat rises, and if you occupy one of the higher floors, the condenser will face additional load trying to cool the hot air. This does not call out for a specific increment in capacity; however, you may want to buy a higher capacity AC if this is the case.
Presence of floor above your premises
This is an important factor and may impact the air conditioning capacity significantly. If there is a floor above yours, the ceiling of your house would not be directly exposed to the sun. If your house is on the top floor, your ceiling would be continuously heated by the sun. If this is the case, increase the base capacity by 25%.
Construction material used for doors, windows, etc.
The material used to construct the place plays an essential role in determining the insulation level. Higher the insulation, heat would take more time to seep into the room. Wood is an excellent thermal insulator. If the door’s made from wood, it will take a lot of time for stray heat to seep into the room. On the other hand, if the door is made from a metal sheet, there’ll be a quick “loss” of cooling.
Points of air leak in the room
Insulation is not just about the construction material. It also means that any points that could lead to the cooler air inside to escape outside should be plugged as much as possible.
Problems with under-sizing or over-sizing
A common thought that many buyers have is to buy an aircon with higher tonnage and avoid going through the hassle of calculating the required capacity properly. Buyers on a budget may prefer going for a smaller-capacity air conditioner. These are cases of oversizing and under-sizing.
An undersized air conditioner would struggle to maintain the room at the set temperature. This would result in a higher compressor ON time, which would place undue stress causing it to wear out faster. With an ambient temperature higher than usual, there is a possibility that the aircon may not be able to achieve temperatures, especially in the lower range. A compressor working ceaselessly would result in very high power consumption leading to increased energy bills. Please check our guide on energy saving tips.
An undersized aircon not only fails to provide the comfort you desire but also consumes excess energy and has a lesser life than an adequately sized aircon.
An oversized aircon is better than an undersized air conditioner, but it also has demerits. While it may cool the room faster, it would require higher capacity electrical components like circuit breakers and wiring. Faster cooling leads to another issue – frequent ON and OFF cycles for the compressor. The aircon’s components, including the compressor, wear out earlier because of higher-than-usual switching. The electrical relay that cuts off the compressor may be the most loaded component in this case and could fail prematurely.
Maintenance cost is higher for larger air conditioners, and replacement parts are priced higher. A larger aircon would also be able to compensate better for sources of heat like electrical appliances, gadgets, open doors, or windows. You’ll end up wasting a lot of electricity because you would not be aware of heat seeping inside the room. Check our guide here on how much it costs to service a aircon in 2019.
The capacity of an air conditioner must be finalized after considering all factors. It is a simple process of considering your landscape, building, room, and other points that may warrant a need for additional capacity. An adequately sized aircon would provide consistent cooling – neither slow nor fast. There would be lesser wear and tear, leading to high longevity. Energy efficiency is optimized when a properly sized aircon is used.
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