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Choosing the right-sized AC unit for your home is essential to ensure your house is comfortable. Only the right size air conditioner will ensure that you are comfortable and not overpaying for your energy. AC units that are too small do not cool your home to a comfortable temperature. AC units that are too big, on the other hand, leave your house feeling drafty or even very hot or humid and use more energy than necessary.

An AC unit’s primary purpose is to provide cool air in your home. However, using a unit that does not match the size of your house will struggle to maintain an even temperature in your residence. The right-sized AC unit will give you excellent results in performance, energy efficiency, and comfort. Therefore, it is essential to understand the basic parameters of an AC unit to determine the proper size for your home.

The primary consideration for choosing the right-sized AC unit is the square footage of your residence.

How to Calculate Size

To effectively determine the suitable AC size you need, you must know the square footage of your house. To calculate the square footage of homes with simple rectangular or square rooms, multiply the width times the length of the rooms in your house, then add them together. If your room has some other shapes, divide the room into sections made up of triangles and squares and take those measurements.

Once you have the total square footage of the space that needs to be cooled, multiply it by 25 to determine the BTUs (British Thermal Unit per hour) required to achieve efficient cooling. Energy Star estimates that you need between 20 to 30 BTUs (British Thermal Unit) per square foot of cooling power.

Air Conditioner Ratings

Another major factor that you need to consider when choosing the right-sized unit is the air conditioner ratings. AC units are usually rated in two ways: cooling capacity and energy efficiency.

Cooling Capacity Ratings

All AC units will have a BTU or BTUh rating. These ratings indicate how much cooling is created by an AC unit by considering the power needed to run an AC unit and the size of the room being cooled.


BTU is a customary measurement used in measuring the quantity of heat energy needed to raise the temperature of one pound of water by one Fahrenheit. In simpler terms, one BTU is the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit.


The BTUh is a more exact measurement of an air conditioner’s cooling capability, known as “tons.” The BTUh measures how many BTUs of heat an AC unit can remove in an hour. Therefore, it evaluates the energy efficiency of an AC unit by calculating the tons of air conditioning dispensed in an hour. One ton removes approximately 12,000 BTUs of heat each hour.

Energy Efficiency Ratings

Energy efficiency ratings are denoted as SEER (Seasonal Energy Efficiency Ratio). SEER ratings indicate the total amount of cooling provided during the cooling season, divided by the total amount of electrical energy used. The higher the rating, the more energy-efficient the unit is.

Some air conditioner systems have an ENERGY STAR certification, indicating that they use at least 8% less energy than ordinary AC models. Such systems are often more energy-efficient and cost-effective to use.

Variables That Might Impact the Size of an AC Unit

There exist different variables that come into play that can impact the efficiency of your AC system to cool your home.

Heat-Generating Appliances

Laundry rooms and the kitchen have appliances that generate heat. Appliances that cause extra heat inside of your home can affect the performance of your AC system, and rooms that house those appliances will require more BTUs to cool down.


In summer, the insulation in your house plays a significant role in keeping cool air in and heat out, and vice versa in the winter. A well-insulated house will require less AC power to maintain comfortable temperatures. However, loose and missing insulation allows heat to enter your home, and air conditioner systems will have to work harder to keep your house cool.

Ceiling Height

Houses and rooms with high ceilings have more volume than those with a standard 9-foot ceiling height. Therefore, you will have to calculate at least two BTUs higher than the recommended standard to efficiently cool the extra room volume.

Shade and Sun Exposure

Homes that face primarily east or north are susceptible to less sun exposure. Therefore, they typically have plenty of shade and are usually cooler. However, west- or south-facing homes generally take longer to cool because they have higher sun exposure during the summer.

Deciding the Type of AC System You Need

There are several different AC system options, each with different features, benefits, and costs, to help cool your home.

Window AC

These units cool one room at a time and require a window. A window AC unit sits on the window ledge and is installed by mounting it to the window frame. Window AC units are easy to install and can be moved, placed, or removed as needed. Therefore, they are convenient for homeowners who live in a smaller space or do not want to spend time wiring an AC unit in the wall.

Portable Air Conditioners

Portable air conditioners are light and easy to carry around with you, ensuring that you can cool different rooms within the same space. This makes them handy tools to use in a specific space or room that needs to be cooled on an as-needed basis. Portable AC units are not as powerful as window or wall AC units. However, they can fit in places where window or wall units would not.

Central Air Conditioners

These AC units cover your entire house with a single unit, allowing you to regulate the temperature in every room of your residence. Central AC units are the most expensive, but they are more efficient and powerful than the window and portable AC units. They are also more costly to install and maintain.

Your Professional Air Conditioning Team

At Sun City Mechanical, we are a family-owned HVAC company that has provided the residents of Sun City, AZ and the surrounding areas with quality plumbing, heating, and cooling services since 2003. We are BBB-accredited, and our technicians are licensed, certified, and always stay current with the latest engineering trends and best practices. We believe in training our technicians to be the best individuals in their field. We also believe in using ethical and safe practices.

Our HVAC technicians can assist you with any questions regarding your heating or cooling needs. Whether you want a new system installed, an AC repair, or a cooling system tuned-up, our team is the best choice in Sun City and the surrounding areas.

If you need residential or commercial HVAC services, we provide emergency repair services 24/7. We are also affiliated with top brands like American Standard, Ruud, and Lennox to provide you with the best service and repair options.

For more information about our AC services or to schedule an appointment, contact us today.