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Air Conditioner Sizing & Buying Guide

An in-depth guide to buying an air conditioner. Learn everything you need to know to stay comfortable when the weather warms up.

Getting Started

Take the edge off of smoldering summer heat with an air conditioner that perfectly fits your needs. Whether you're in the market for a small window unit or a whole-house central air system, you've come to the right place. This guide covers BTUs, Voltage, Styles, and Features in detail to make sure you find the perfect air conditioner for your home.

Size

A common misconception about air conditioners is that bigger is better. However, oversized AC units will leave the space they cool feeling cold and clammy. As air conditioners lower temperatures, they also remove moisture from the air. When an AC's cooling capacity matches the room size, it will effectively manage both temperature and humidity.
Getting an air conditioner that's too small for the space comes with a different set of issues. Most importantly, it will struggle to lower temperatures. As a result, the unit will run constantly, wasting power as your utility bill rises.
To avoid these types of issues, the first step to buying an air conditioner is measuring the space you want to cool. Room sizes are measured in square feet. To find the square footage of your room, simply measure the length and width of the room and multiply the two numbers. Once you know how large your space is, check the chart below to see what size AC you need.
Room Size (Sq. Ft.) Room Size (Sq. M.) BTU
100 up to 150 9 up to 14 5,000
150 up to 250 14 up to 24 6,200
250 up to 300 24 up to 28 7,000
300 up to 350 28 up to 33 8,000
350 up to 400 33 up to 37 9,000
400 up to 450 37 up to 42 10,000
450 up to 550 42 up to 51 12,000
550 up to 700 51 up to 65 14,000
700 up to 1,000 65 up to 93 18,000
1,000 up to 1,200 93 up to 112 21,000
1,200 up to 1,400 112 up to 130 23,000
1,400 up to 1,500 130 up to 140 24,000
1,500 up to 2,000 140 up to 186 30,000
2,000 up to 2,500 186 up to 232 34,000
As you can see, the effectiveness of an air conditioner is measured in BTUs (British Thermal Units). A BTU is defined as the amount of energy required to increase or decrease the temperature of a pound of water by one degree Farenheight. The BTUs outlined in the above chart are a good starting point, but issues like sun exposure can make a big difference.
  • Especially sunny rooms will need roughly 10% more power.
  • Rooms that get little to no sun will need 10% less.
  • If the room is regularly occupied by more than two people, add 600 BTUs for each additional person.
  • For kitchens, add 4,000 BTUs.

What To Know Before You Buy An Air Conditioner

Types of AC Units

After figuring out the right size for your unit, you need to determine which type of air conditioner makes sense for your space.

Window Air Conditioners

Window Air Conditioners
The most common room air conditioners are window units. Designed for double-hung windows, the kind that opens by sliding the bottom half up, window ACs are easy to install and easy to remove come fall.
Just slide the window open, rest the AC on the sill, close the window, and secure it in place. If the air conditioner isn't as wide as your window, most include side curtains to fill the gaps. While it's common to put window ACs in storage when the weather cools, they can also be installed permanently.
Window air conditioners pull in the hot air from your room and run it over a cooling coil before returning blowing it back into the room. Passing the hot air over the cooling coil also pulls moisture from the air, creating dew. The dew is funneled outside, as is the heat produced to keep the cooling coil cold.
Innovations in designs have greatly improved the energy efficiency of window air conditions in recent years. From high-efficiency compressors to energy-saver settings, these improvements help you save money while staying cool. Look for ENERGY STAR certified models to cut down on electricity costs. Even better, compare the Energy Efficiency Ratio (EER) or the Seasonal Energy Efficiency Ratio (SEER) of any models you're interested in. The higher the number, the more efficient the air conditioner.
The SEER details a model's average efficency over a range (65°F to 104°F) of outside temperatures. The EER bases its figure on a steady outside temperature of 95°F. If you primarily use your AC when outside temperatures are 95°F or higher, the EER will be a more accurate figure. Otherwise, look to the SEER. Don't worry if the model you're interested in only providers one or the other. Either number will still give you a general sense of the model's efficency.

Casement Air Conditioners

Casement Air Conditioners
A casement air conditioning unit is designed for windows that open horizontally or swing-out (usually with a hand crank) instead of vertically. Casement ACs are basically taller skinnier window units.

Portable Air Conditioners

Portablesement Air Conditioners
If you can't mount an AC to your windows, a portable air conditioner might be the best option. These models usually rest on the floor and use a flexible hose to vent hot air outside. The venting hose can usually fit through a small space, such as the center vent often found in basement glass-block windows.
While it can be convenient to roll your AC between rooms, portable air conditioners have their share of drawbacks. Most notably, they're less efficient and noisier than window units. While window models can project their noise outside, a portable air conditioner doesn't have that option. Similarly, portable ACs need to spend energy pushing excess heat outdoors while window units can let that heat passively radiate out.
Portable air conditioners also remove moisture from the air just like window units. But, unlike window units, they can't just drip the moisture outside. Most portable air conditioners collect the water in a tank that must be manually emptied from time to time. Self-evaporating models need to be emptied less often but aren't enitrely hands-off. Other models allow you to attach a drainage hose to manage the water it collects.

Wall Air Conditioners

Wall Air Conditioners
For a more permanent solution, consider a wall air conditioner. These models extend from a wall instead of a window and require a sleeve to be properly installed. Often more efficient than window models, through-wall air conditioners are a good way to stay cool without sacrificing one of your windows. However, installing an in-wall AC isn't as easy as installing a window unit. Unless you're comfortable cutting holes in your wall, hire a contractor. If you're in the Chicagoland area, we here at Abt would love to help!
Packaged Terminal Air Conditioners (PTACs) are in-wall units that can heat and cool your space. In-wall units are left in place year-round. Having one appliance to warm you up in the winter and cool you down in the summer just makes sense. If winter temperatures even approach freezing where you live, be sure to get a PTAC with electric heat. The alternative, heat pumps, are less expensive. But they're also less efficient.

Central Air Conditioning Systems

Central Air Conditioning Systems
If you're looking for a whole-home solution, Central Air Conditioning Systems are the way to go. Usually, one main unit pumps cool air through ducting to each room in the building. While these systems are more expensive than the other options, they are much more convenient. Upfront and energy costs are both higher with a Central Air Conditioner. Because instead of using room air conditioners to manage specific areas of your home, central air keeps your whole house cool.
Central air systems with zone controls combine the convenience or a whole house system with the localized controls of window and portable ACs. These systems include thermostats in each room so that you can precisely manage temperatures anywhere in your home. This versatility can be especially useful for poorly insulated or especially sunny rooms.
If you are looking to do away with portable or window air conditioners, Abt is licensed to install central air systems. This service is only for our Chicagoland customers. Give us a call at 800-860-3577 so we can find a time to come look at your home and provide an estimate.

Mini-Split Air Conditioning Systems

Mini-Split Air Conditioning Systems
Mini-split AC systems are electric heating and cooling systems that do not require ductwork. They consist of an indoor unit and an outdoor compressor. The indoor units install above a doorway or just below the ceiling. These highly energy-efficient devices are a great whole-home climate solution. By installing indoor units in every room, each member of your family can dial in their ideal temperature for maximum comfort. Depending on the model, one compressor can control up to eight rooms.

Other Things To Consider

Once you've decided which type and size AC unit to buy, you need to figure out which features you want in your air conditioner. Features like AC controls, remote controls, as well as integrated air filters are all key things to consider.

Ability to Direct Air Flow

Even the most powerful air conditioners won't be much help if they can't direct the cold air where it's needed. With this in mind, most models use adjustable louvers that allow you to direct airflow vertically or horizontally.
When selecting an AC, consider your room layout. Directional airflow is especially important if you plan on installing the air conditioner in a window that sits behind a couch or other obstruction. Make sure to buy an AC unit that can direct the cool air above or around any obstructions.

Controls

The most basic air conditioning units will have very simple controls: a knob for temperature and a knob for fan power. These models are great for getting a blast of cold air into a room but don't allow for subtle climate controls. More advanced models are equipped with digital controls that can monitor and maintain precise temperatures.
Remote controls are another convenient feature to consider. With a remote, you can make temperature adjustments from anywhere in the room. Keep it on your nightstand and you'll be able to cool things down at night without even rolling out of bed.
Many air conditioners can be programmed to turn on or off at preset times of the day or week. This money-saving feature makes it easy to have a comfortable house to return to after work without wasting electricity while you're gone. Just program it to turn off before you leave in the morning and then kick back on a bit before you come home.
Some newer air conditioners even offer smart controls. These models can be adjusted with an app on your smartphone from anywhere. Many smart ACs will even respond to voice commands via digital assistants like Amazon's Alexa or Google Assistant.

Heat

A few models provide heat as well as AC. This is a great option for those times between seasons when it's a little chilly, but not cold enough to turn your home's furnace on.

What's my plug type and what voltage do I need?

Your plug type will let you know the voltage and amperage requirements of your new AC. Make sure your new AC Unit matches your existing electrical requirements. Attempting to plug your AC unit into an outlet with a different plug type or voltage can be very dangerous. Below is a chart illustrating plug types and their corresponding electrical information.
Voltage and Amperage
Edited By Abt Sales Specialist Shirley H. on 6/10/2020

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