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Have you been asking yourself “what size air conditioner do I need?” Then you’re in the right place! We’ll answer this question and many more in this post.

If it’s time for you to install an air conditioning unit in your home – or maybe even just in an individual room – there’s a lot you’ll want to consider. Beyond your individual cooling needs (and your budget concerns), you’ll also want to think about what’s practical for the size and shape of your home or room. Is bigger always better? Will getting a bigger unit mean my home will get cooler faster? You might be surprised by some of the answers. 


Wondering “What Size Air Conditioner Do I Need?

Choosing the wrong-sized air conditioning unit could ultimately hurt your wallet when it comes down to your monthly electric bill. You might also end up over-extending the capacities of your unit. Or, creating maintenance concerns and shortening its lifespan.

Finally, you might end up overpaying for a large unit that you don’t need. This will ultimately cause humidity concerns in your home or room and could leave you feeling uncomfortable. There are a lot of things that can go wrong when your air conditioning unit isn’t appropriately sized for the space. This is why it’s so important to sit down with a professional air conditioning installer to learn about what size unit might be appropriate for your needs.

Today we’ll look at some key factors you’ll want to consider when asking yourself “what size air conditioner do I need for my house.” We’ll also cover some big mistakes you’ll want to avoid when making that choice. 


8 Factors When Calculating Your Ideal Air Conditioner Size

When you ask yourself “what size air conditioner do I need” there are a lot of factors that come into play. Let’s talk about the eight key things you’ll want to consider when making that determination. 


The Size of the House

Wondering ‘how do I calculate what size air conditioner I need?’ It’s actually pretty easy! Most industry experts will tell you to multiply the square footage of the room or home you’re cooling by either 20 or 25: this is the number of BTUs that your cooling unit should provide you.

What are BTUs, you ask?

BTU stands for ‘British Thermal Units.’ It’s the standard unit used to establish how much an air conditioning unit will be able to cool a room or space within an hour. (BTUs are the standard measurement unit, no matter if you’re wondering ‘what size central air conditioner do I need,’ ‘what size portable air conditioner do I need,’ or ‘what size whole house air conditioner do I need.’

In fact, here’s a quick ‘what size air conditioner do I need calculator:’

Width of room * length of room * 25 = approximate BTUs. 


The Number of Windows

The windows you have in a room will definitely have an impact on your AC’s efficiency (especially if you live in a particularly sunny place).

If your room has a lot of windows and is therefore subject to a lot of direct sunlight, you’ll want to increase BTUs in your calculation by at least 10%. Something else to keep in mind is the quality of those windows. If your home does not have that many windows, but those windows are 40 years and old – and single-pane – you may still want to increase BTUs by 10% in calculations. That’s because your windows are not as efficient when it comes to heat transfer. 


The Type of Insulation in Your Home

A well-insulated and more efficient home will not allow exterior heat to transfer into the space as much as a poorly insulated home will. This is something to keep in mind when calculating the size of the air conditioning unit you’ll need to cool it.

If the type of insulation in your home is highly efficient, you might be able to get by with a slightly smaller unit. If your home is older, poorly insulated, or the insulation is of poor quality, you should err on the side of a larger unit because heat will likely transfer more easily from the outside. 


The Height of Ceilings

The square footage of a room is a great help in determining the size of the space that needs to be cooled. But it doesn’t paint a whole picture. Your room – or home, is three-dimensional, after all. So, height will matter greatly when determining the amount of space that will need to be cooled. A bedroom with eight-foot ceilings would be far easier to cool, for example, than a living room with 20-foot cathedral ceilings. 

Keep this in mind when you’re calculating BTUs based on square footage. Because ultimately, not all square feet are created equal). 


Heat Transfer Between Walls

We talked about the quality of insulation of your home and its exterior walls… but what about the interior? Are you looking at installing a portable air conditioning unit in just one room of your house? Then you need to take into account how much heat transfer – if any – will be occurring between rooms. Does the room you’re cooling back up to a poorly insulated kitchen with an oven you use often? This impacts your unit’s ability to keep that room cool. 

So, you might consider increasing BTUs so your unit is not overly taxed. 


Your Average (and maximum) Summer Temperatures

The quality of insulation of your exterior walls matters, your number of windows matters, your direct sunlight exposure matters… but the degree to which all of these things matter in determining your air conditioning unit size will vary depending upon where you are. 

In Phoenix Arizona, your summers will get well into the high 110’s and your direct sunlight will be constant. If you’re in northern Montana, the severity of your summer weather will be far less. That means you can stand to skimp on your BTU calculation a little. Bear that in mind when you’re figuring out the ideally-sized unit for your space, because environmental factors will, unfortunately, have a serious part to play. 


The Color – And Type – Of Roof That You Have

Unfortunately, when it comes to heat transfer and insulation, not all roofs are created equally. Some are simply better at trapping cool air in. Asphalt shingled roofs (for example), are not always the most efficient, especially if they’re older. On the other hand, newer metal roofs are surprisingly adept at keeping homes cool. Talk to your air conditioner installer about the roof you have. 

Find out whether or not it will help – or hurt – your air conditioner’s ability to keep your home cool. 


Does Your Roof Have Ventilation?

In the interest of keeping your cooled air in, you might think that a well-ventilated roof would be a problem. In fact, you want to have a well-ventilated roof. Cool air sinks, and hot air rises. So that hot air that you don’t want in your home will float to the top of your home, and (if your roof is well-ventilated) escape, keeping the cold air inside where it should be. 

It’s worth it to find out how well-ventilated your roof is and keep this in mind when you’re looking to install a new air conditioner. 


What Size Air Conditioner Do I Need For My Home? Common Mistakes

Knowing what we know now about the size, build, windows, and roof of your home, it might be easy to figure out how to determine air conditioner size. But there are a couple of key mistakes that a lot of new homeowners make, and these can be quite costly in the long run. Let’s explore what not to think when calculating the ideally-sized air conditioning unit for your space. 


“A Larger Air Conditioning Unit Can’t Hurt, Right?”

This is a common misconception and one that can leave you feeling really uncomfortable. Air conditioners work by extracting moisture (humidity) from the air. But an oversized unit working in too small a space will draw so much humidity from the air that it’ll backfire. In the end, it will actually make a space feel more humid. You’ll be cool… and clammy, chilly, and ultimately too cold. 

Trust us: the feeling is not pleasant. And not worth it! 


“An Undersized Air Conditioner Would Save me Money.”

This is also a fallacy. Buying a unit that’s too small for space will cause that unit to have to seriously overwork itself. Likely, space will never cool down to quite the degree that you want it to. But you’ll also be creating added stress on the machine. This leads to costly breakdowns and malfunctions far too early in the life of the unit. 

If you really want to benefit your wallet, choose an appropriately-sized machine for your space that won’t get overworked. 


Still Wondering “What Size Air Conditioner Do I Need?

When it comes to asking ‘what size air conditioner do I need for my home,’ there is a lot to consider. But if you think critically about the size of the space you’re trying to cool, the building composition (windows, roof, and ceiling height) of the space, and how well insulated the building is, you should be able to figure out what unit will work best. 

And if all else fails, talk to some experts! The knowledgeable staff at A/C Care are here to consult on what air conditioning unit will best suit your unique needs. Give us a call to find the A/C that’ll work for you. 

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