Aircond HP Calculation: How Many HP Do You Need?

Air conditioners or airconds are becoming a necessity in Malaysia with the ever-increasing hot weather. If you’re getting a new aircond for your house, you’ll need to know how many HP you need for your bedroom, living room and other rooms. In this article, I’ll guide you on how to calculate aircond HP.

In Malaysia, a 1.0 HP aircond is usually sufficient for a 100 to 150 square feet room. For a 150 to 200 square feet room, you’ll need a 1.5 HP aircond. A 2.0 HP aircond is adequate if you have a 200 to 300 square feet room. If you have a 300 to 400 square feet room, a 2.5 HP aircond is needed.

Here is a summary of aircond HP vs room size (in square feet):

For most people, the above recommendation is all they need to decide what aircond HP to buy. However, if you have special cases such as tall ceilings, large windows/sliding doors, family gathering needs, or you want to save money, it is necessary to understand how aircond HP is calculated so that you can make an informed decision.

Aircond HP Simplified Calculation

In Malaysia, most people calculate aircond HP using what’s called the Rule of Thumb. It is the “how many btu/hr per sqft (square feet)” approach. The magic number is 60 btu/hr per sqft.

This approach is mostly applicable, especially for typical house layouts. Besides, it is easy to understand and everyone can apply the formula. Therefore, this approach to sizing aircond is widely adopted.

Given a room that is 10 ft wide and 15 ft long, the floor area of the room is 150 sqft. Using the magic number of 60 btu/hr per sqft and multiplying it by the room floor area of 150 sqft, we get the aircond size at 9000 btu/hr.

150 sqft x 60 btu/hr per sqft = 9000 btu/hr

By right, the British Thermal Unit per Hour or btu/hr is the “official” cooling capacity of air conditioners. This measurement unit can be seen on all air conditioner catalogues.

However, we Malaysian prefer to use HP which stands for horsepower.

Without going into the history of HP and btu/hr, the approximate conversion is 1.0 HP equals 9000 btu/hr. Sometimes, 8500 btu/hr is also categorized under 1.0 HP but 9000 btu/hr is mostly the case.

So, the 150 sqft room requires a 1.0 HP aircond.

We can repeat the same calculation for other room sizes. For example, 300 sqft times 60 equals 18000 btu/hr. Since 9000 btu/hr is equal to 1 HP, 18000 btu/hr is 2 HP. So, a 300 sqft room needs a 2.0 HP aircond.

Ceiling Height Above 10 Feet

By default, this method (using 60 btu/hr per sqft) assumes the ceiling height is 10 ft or below which is quite typical for Malaysian homes. But, if your ceiling height is more than 10 ft, then for every extra foot, add 10% to the btu/hr.

For example, if the 300 sqft room has a ceiling height of 13 ft, then 18000 btu/hr times 1.3 (extra 30%) equals 23400 btu/hr. Since 9000 btu/hr is equal to 1 HP, 23400 btu/hr is 2.6 HP.

Suddenly, the same room now requires a 2.5 HP aircond.

It makes sense because the room is technically bigger with a taller ceiling. There is more air in the room to be cooled. So, the 10% addition to every foot above 10 ft ceiling height ensures the aircond is adequately sized.

Prolonged Exposure to Sunlight

Rooms that are located at the corner of the house, especially corner landed houses, semi-D and bungalows have prolonged exposure to sunlight. This adds extra heat to the room which the aircond must remove to cool.

A general rule to accommodate this extra heat is to increase the aircond HP by one size.

This is critical if a big part of your room is directly facing the east and west directions as these are the two directions where you’ll get the most direct sunlight.

Furthermore, if you have a large sliding glass door, especially in the master bedroom where nothing is blocking the sunlight from entering, it is also advisable to up the aircond HP by one size.

The standard aircond HP has 4 sizes:

• 1.0 HP
• 1.5 HP
• 2.0 HP
• 2.5 HP

If you have calculated that you need 1.5 HP but you notice the room is receiving a lot of sunlight, then you might want to consider using a 2.0 HP aircond to ensure the aircond is still cold enough in a very sunny day.

About Oversizing and Undersizing

Nowadays, most air conditioners are inverter type, meaning they can alter their compressor speed based on how much cooling is needed. So, oversizing your aircond by one size is not a problem at all.

On the other hand, undersizing can be troublesome. While inverter air conditioners can run at 105% of their rated capacity (eg: 12000 btu/hr runs at 12600 btu/hr), it mostly likely still won’t be enough during hot days.

So, we generally prefer slightly oversizing the aircond rather than undersizing it.

Modern inverter air conditioners can halve their cooling capacity when not needed (eg: 12000 btu/hr ramp down to 4000 btu/hr). This feature not only saves energy but also reduces the workload of the compressor which can prolong its lifespan.

However, if you have a large room and you find that the aircond HP needed exceeds 2.5 HP, it is recommended that you get a professional to help because the larger the aircond HP, the riskier it is for you to use the Rule of Thumb method. You may oversize the aircond too much which can lead to high humidity issues.

Aircond HP Calculator

With a basic understanding of the aircond HP calculation, you can use the below calculator to quickly find the required HP for a given floor area and ceiling height:

The above aircond HP calculator provides an estimation based on 60 btu/hr per sqft. It also accounts for intense sunlight but refrained from oversizing. Use this result as a reference and cross-check with your aircond contractor.

How Aircond HP is ACTUALLY Calculated?

When it comes to complex buildings like office towers, hotels and shopping malls, while some engineers still use the Rule of Thumb method to calculate aircond HP, the proper way is what we call a cooling load calculation.

Cooling load calculation is a complex procedure where we gather information such as the thickness of the wall, material type, window glass thickness, roof insulation, lighting power and the number of people expected to be staying in the room.

Then, we use mathematical formulas to calculate how much heat is flowing into the room through the wall, window, door, roof and floor. Next, we include the heat gain from the lights, people and any equipment operating in the room.

Finally, we do a psychrometric analysis with the help of charts and software to size the air conditioner for it to achieve the indoor temperature and humidity we want.

As you can see, the “proper” way of sizing the air conditioner is lengthy and difficult. Even with software nowadays, the amount of inputs needed still takes a lot of time to gather.

Meanwhile, the Rule of Thumb method of using the floor area and multiplying 60 is straightforward. Over the years, rooms sized this way have performed relatively well. Hence, this method is still widely used, even among consulting engineers.

FAQs When Buying an Aircond

Buying a new aircond is more than just knowing how many HP. There are many options you can choose. Here are some of the frequently asked questions about air conditioners:

Is Inverter or Non-Inverter Better?

Inverter air conditioners are better if you use them for long hours (eg: 4-5 hours every day). The longer you use the air conditioner, the more savings the inverter can give you. Plus, the room temperature is more consistent with inverter ACs which means more comfort.

Can I Save Electricity with Smaller HP Aircond?

To save energy, look for air conditioners with the highest CSPF. If you’re deciding between a 1.5 HP aircond with 4.60 CSPF and a 2.0 HP aircond with a 5.02 CSPF, the 2.0 HP aircond will consume slightly less electricity giving the same situation.