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Home > Projects & DIY Toolkit > Buying Guides > How to Buy a Room Air Conditioner

How to Buy a Room Air Conditioner

For efficient cooling of smaller spaces, a room air conditioner is a great choice. Today's energy-efficient models can also handle larger areas with ease. The key to choosing the right air conditioner is matching the proper unit to the space you want to cool. Lowe's is happy to provide this guide to buy air conditioners as a service to you.

The technology is the same as your refrigerator. An evaporator coil cools the interior (in this case the room) and the condenser coil releases the hot air outdoors. Refrigerant is moved by a pump through tubes between the two coils to transfer the heat. A fan blows the cool air into the room.

Guide to Buy an Air Conditioner - Terminology and Product Features

Choosing the right air conditioner is easy with the right information. The first step before you go shopping, is to learn to speak the language.

Btu - The British Thermal Unit (Btu) is the international measurement of energy. Here's the definition: a Btu is the amount of heat required to raise one pound of water one degree Fahrenheit. In AC jargon, Btu's measure the amount of heat an air conditioning unit can remove from the room. As the Btu rating increases, so does the size, weight and cost of the air conditioner. Room air conditioners are generally 5000 - 24,000 Btu's. Matching Btu requirements to room size is important. The room(s) won't cool efficiently if the Btu rating is too low or too high for the size of the room.

EER - The Energy Efficiency Ratio is calculated by dividing the Btu's by the number of watts. The higher the EER number the more efficient the unit should be. A higher EER rated air conditioner will also be more expensive, so decide if the higher price tag is worth the energy savings you may be getting. The ratings range from 8-11.5. If it's really hot where you are, go for a rating of 10 or higher.

Thermostat - The heat-sensing thermostat adjusts the temperature of the air coming from the air conditioning unit. They can be programmable or manual. An adjustable thermostat combined with a variable-speed fan is an effective cooling machine. AC units are available with such features as remote control, 24 hour delay timers and digital temperature readouts.

Filter - Clean air is essential for efficient operation. Look for units with filters that can be easily cleaned or replaced.

Fan - Here's the thing that moves the air. A variable speed fan combined with an adjustable thermostat is an effective cooling machine. Look for louvers that adjust up/down and right/left.

Chassis - A fixed chassis is characteristic of a lighter weight, smaller unit that's easy to remove for winterization. A slide out chassis is found on larger, heavier models. The unit slides out easily for maintenance.

Before You Buy

Measure the Room

Determine the square footage of the room (multiply length x width).

There are lots of variables to consider when choosing a room AC — number of windows, number of people in the room, which direction the room is facing, ceiling height, etc. Energy Star has this chart on finding the right size room air conditioner to meet your needs.

Area To Be Cooled
(square feet)
Capacity Needed
(Btu's per hour)
100 to 150 5,000
150 to 250 6,000
250 to 300 7,000
300 to 350 8,000
350 to 400 9,000
400 to 450 10,000
450 to 550 12,000
500 to 700 14,000
700 to 1,000 18,000

Remember – bigger is not better. A unit that’s too large will cycle on and off too frequently, using too much energy and causing unnecessary wear on the electrical components.

Measure the Window Dimensions

Most window air conditioners are designed to fit double-hung windows. Measure the inside dimensions (the lower part of the window that raises). Take an exact measurement, not just an estimate.

Take your room dimensions and window measurements with you when you go shopping.

Check the Power Supply

What kind of plug does the unit have? What kind of plug is your wall outlet? Make sure the circuit can handle the operation of the unit. Smaller ones work with a standard 115-volt outlet. Larger 115-volt units may need a dedicated circuit. The largest AC units require a 230/208-volt circuit.

Other Things to Remember


How heavy is it? Get some help when carrying and installing the unit. An AC unit can easily weigh over 100 pounds.


Although newer units are much quieter than older models, be prepared for some additional noise in the room.


Follow the manufacturer's instructions carefully when installing a window unit. It's essential that the unit be stable for safety reasons. It's also essential that the unit be tightly sealed to keep the cool air inside where it belongs. Apply weatherstripping as needed to prevent airflow around the unit. Put the AC in a shaded window if possible for added cooling efficiency.

SAFETY NOTE: When adding a new circuit, always have a licensed electrician connect the power to the panel box. Also make sure your air conditioner is properly grounded. Never alter a plug to fit your outlet.


Air conditioner problems are easily avoided by doing some simple maintenance. Keep the filter and coils clean or replace them as recommended by the manufacturer. Don't need a new AC just yet? Here's some pre-season maintenance advice for your old unit:

Remove the inside grill. Carefully take out the foam filter. Wash in warm soapy water. If the filter's extremely dirty, more than one washing may be needed. Rinse and allow the filter to dry before replacing. Filters that are beyond the reusable state can be inexpensively replaced.

Filter maintenance alone will increase your air conditioner's efficiency, but to get the most from your unit, a thorough cleaning is required.

Take the unit from the window and remove the outside housing. Vacuum the interior to remove dirt, bugs and leaves. Cover electrical parts with plastic and then wipe the interior with a damp cloth. While the unit is out of the window, take time to clean and repair the window and sill if necessary. Reassemble when dry and reinstall in the window. Reseal around the unit with new weatherstripping.

SAFETY NOTE: Always unplug the unit before doing any maintenance.

Tools, products, materials, techniques, building codes and local regulations change; therefore, Lowe's assumes no liability for omissions, errors or the outcome of any project. The reader must always exercise reasonable caution, follow current codes and regulations that may apply, and is urged to consult with a licensed professional if in doubt about any procedures. Please visit our terms of use.

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