air conditioner

The changing of the season means an increase in temperature, both outside and in. Mother nature is in control of how hot it gets outside, but you can keep your cool inside when you have an air conditioner. If you’ve just started looking, you might have noticed that not all cooling units are the same. To cool your space properly, you have to choose the right size.

How do air conditioners work?

sizing a window air conditionerAir conditioners are made up of an evaporator, compressor, condenser, and expansion device. They work by using a chemical inside the unit that cycles inside and outside to remove heat and replace it with cool air.

There are two types. You can find window air conditioners that sit inside the frame of your window or portable air conditioners that you can move from room to room.

If you plan on choosing a portable air conditioner there’s one thing to note: portable units include an exhaust hose that will vent warm air and moisture, so you’ll want to ensure you have a window nearby to drape the hose out of.

What you should consider before you buy an air conditioner

sizing portable air conditionerThere are a few things to keep in mind when you’re choosing the best size of cooling unit for your home. The first is that the AC will remove heat from the air, but it will also reduce humidity. That’s why it’s so important to take into account exactly what size of room you’d like to cool.

If you choose an oversized unit it will cool your room more quickly, but it won’t do a good job of removing the humidity. Your room could feel damp and clammy, and if the room is damp for long periods of time you could end up having an issue with mold and fungus.

Calculate the size of your space

Take a look at the room you’d like to use the air conditioner in. Most rooms are rectangular, so to get the measurement it’s simply length times the width of the room. If you have an odd-shaped room you may have to calculate the square footage by splitting the room into rectangles and triangles, then taking the area of each and adding them up.

Once you know the square footage of your room you can look for a size that will best suit the space.

BTU and ASHRAE labels for portable air conditioners

portable air conditioner ratings

If you’ve already shopped for an AC, you know the cooling capacity has always been measured in BTUs (British Thermal Units). BTU is a term to measure heat, and the higher the BTU rating, the more efficient the cooling power.

Recently, the US Federal Trade Commission (FTA) and the Department of Energy (DOE) changed the testing and labeling for air conditioners. The new DOE labels and rating system is designed to more accurately reflect the energy efficiency of each portable air conditioner unit.

The new changes can seem a bit complicated, but the overall idea couldn’t be simpler. Both ASHRAE labels and BTU labels focus on the number of BTUs removed from the room by your air conditioner. However, the new DOE labeling takes into account something that ASHRAE does not; the portable air conditioner itself can also generate heat. Because of that, the new ratings will lower the effective cooling power of the unit. For example, a portable air conditioner rated at 10,000 BTU under ASHRAE will now lower to 6,500 BTU under the new labeling.

As of right now, portable air conditioners at will only show the older rating system of BTU. Over time, you’ll notice that both ratings will appear in the product descriptions.

What should your air conditioner BTU be?

BTU Chart to size portable ACNow that you know about BTUs, ASHRAE ratings, and why they’re important, you can calculate the size of your air conditioner.

The first step in determining which BTU is for you is to calculate the size of your space. The next step is to use the following table to determine the standard BTU sizing for the room you want to cool (note this is based on the old method, so you might want to go up a few sizes if the unit shows the ASHRAE rating.)

What to take into account when choosing BTU

To get the most out of your unit and ensure it will cool the space you want to cool, you need to be as close to the right BTU as possible. That means taking into consideration the amount of extra heat in the space you are cooling.

These are a few adjustments you should make when selecting the right BTU.

  • If the room you’re cooling has constant sun, add 10%
  • For heavily shaded rooms, reduce by 10%
  • In a kitchen, add 4,000 BTUs
  • If the room has high ceilings that are 10 feet or higher, add 10%
  • For rooms that are regularly occupied by more than two people, add 600 BTUs for each person above two

What is an Energy Efficiency Rating?

black portable ACThe last thing you should consider is the EER (Energy Efficiency Rating) of the air conditioner. The EER lets you know how energy efficient the air conditioner is to operate. EER rating is simply calculated by dividing the BTU of an air conditioner by its wattage.

Air conditioners with a high EER rating will use less electricity to create the same amount of BTU’s of output. Another benefit of a higher EER is it likely has a better compressor and runs more quietly than other models.

Ready to cool off?

Now that you know what you need to consider when buying a unit to cool your home, you’re ready for a long, hot summer. Check out all of the air conditioners and fans available on Best Buy from brands like Danby, Haier, De’Longhi, Insignia, and Honeywell and covering all types. Check out our comprehensive air conditioners buying guide for more tips as well, and to help you choose between portable and window AC.

If you’d like to know how to install your air conditioner, take a look at this post.

Choose from portable air conditioners you can wheel from room to room or window air conditioners to keep your bedrooms cool and enjoy a cool breeze when the temperatures start to soar.

Editor TV & Home Theatre
I'm a Vancouver freelancer and tech enthusiast. When I'm not writing you'll find me on my farm with my alpacas, chickens, and honeybees. Visit my website Survivemag


  1. I live in a three bedroom apartment building. I am confused as to what air-condition to buy.

    3 air conditioners or one? How much hydro am I expected to pay if I have three air conditioners?

  2. The portable units still need to be vented to the outside. You should not run them without the exhaust hose being vented out a window.

  3. These portable air conditioners are designed to work inside without direct access to outside ventilation. They will work well placing them anywhere inside.

    • This is not true! All of the portable units Best Buy sells need a window to exhaust the hot air (and sometimes the water vapor depending on the model). You cannot just place them in the middle of a room and expect any cooling! OMG If there is no window, you can create a hole through the wall to the outside. But you must vent to the outside in some manner.

  4. Do this portable air conditioner “not window units” require outside ventilation, please let me know

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