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Here’s how to calculate what size of air conditioner you need

Need a new air conditioner this summer? It’s critical that you get the right size of air conditioner for your home. In this air conditioner sizing guide, we’ll walk you through what you need to know about finding the right-sized system.

We’ll review how AC size is measured, why a bigger air conditioner isn’t always a better choice, and why you should schedule a free in-home estimate with our team.

Your AC sizing guide

Contrary to popular belief, air conditioners aren’t one-size-fits-all. Different homes require different air conditioners with different capacities.

If you’re looking to upgrade to a new system, you’ll need to work with a professional to figure out exactly what size of air conditioner your home needs.

In our air conditioner sizing guide, we break down what factors you need to consider before purchasing a new AC unit.

Understanding BTUs

The output of an air conditioner is measured in British Thermal Units (BTUs). Used in a wide variety of sciences and industries, BTUs measure how much energy is needed to heat a pound of water by one degree. For our purposes, when discussing air conditioners, they measure how much energy is required to remove heat from the home. The higher a system’s BTU output, the more powerful it is.

When comparing different air conditioners online, you may not see the BTU output listed alongside other information about the system. There’s a reason for that: most manufacturers create different BTU versions of the same model, which allows experienced HVAC techs to match homes to the right-sized air conditioner.

Air Conditioning Tonnage

To muddy the waters even further, not all air conditioners are measured in BTUs. You’ll sometimes see them measured in air conditioning tonnage, or “tons.” The simplest conversion here is that every “ton” equals 12,000 BTUs. A 36,000 BTU system is a 3-ton system. As with BTUs, the higher the system’s tonnage, the higher its cooling capacity.

Why are air conditioners measured in tons? The answer dates back to the earliest days of refrigeration, when people transported large blocks of ice for cooling. A refrigeration ton is the rate at which a ton of ice (literally 2,000 lbs!) either melts or freezes at the freezing point over the course of 24 hours.

12,000 BTUs = 1 Ton = 3.5 kW

You’ll often see tonnage used to measure larger air conditioning systems, with BTUs used in the context of smaller systems where the output would be less than a single ton.

AC capacity versus physical size

One final note: when discussing “air conditioner size,” we’re not referring to the physical size of the air conditioner. With few exceptions, there’s often no correlation between BTU output and the actual, tangible size of the condenser unit that’s in your backyard or mounted to your roof.

How do you calculate BTU for a room?

There’s no simple formula for this. While it’s true that your home’s square footage is included in this measurement, it’s far from the only factor that needs to be taken into account.

To find your home’s BTU needs, HVAC technicians also have to consider factors like the number of windows, north- and south-facing walls, the number of stories, the type and quality of insulation, and much more.

This is why most HVAC companies—including ours!—offer free in-home estimates on new air conditioners.

During your free estimate, our installation expert will take detailed measurements and notes, allowing us to quote you for the right-sized air conditioner. You wouldn’t buy new kitchen cabinets, or get new windows, without first having an installer take detailed measurements. The same principle applies here, too.

Is it better to get a bigger air conditioner?

On paper, it seems to make sense. Why not get the biggest air conditioner for your money? However, when it comes to air conditioners, bigger isn’t always better.

It’s true that larger air conditioners are capable of a higher BTU output, cooling larger homes faster. However, when the system is too large for the home it’s in, it’ll run into major performance and efficiency issues.

To operate efficiently, air conditioners need to start up and then settle into a steady, maintainable speed. This allows the air conditioner to cool without wasting energy, and it’s why variable-speed systems—which essentially have different operating speeds they can shift into—are able to use far less energy than traditional single-speed air conditioners.

An air conditioner that’s too large is too overpowered for its own good. In a home that’s too small for them, they’ll start up, cool the home at full power, and then shut off. This is known as AC short cycling.

This cycle will repeat itself constantly, while the air conditioner uses more power and experiences more wear-and-tear with that constant starting-and-stopping.

Your home’s comfort will yo-yo back-and-forth between hot and cold, while your air conditioner struggles to get enough runway to operate efficiently.

What if my air conditioner is too small?

As you may have already guessed, this is also not ideal. An air conditioner that’s too small—again, referring to BTU output, not physical size—will struggle to keep up with your home’s cooling needs. Unlike the bigger air conditioner, it will run constantly, fighting to keep its proverbial head above water. All this leads to significant energy costs and extra wear-and-tear on your system.

If you’ve already had such a system installed in your home, you’ll either need to replace it with an appropriately sized system or supplement its cooling capacity with the addition of a mini-split system. In some cases, our team can help by adding attic or wall insulation, which reduces the total cooling capacity needed in your home.

However, keep in mind that factors like square footage, the number of stories, and the number of windows are often the key determining factors in AC sizing, and there’s not much you can do to change those aspects of your home.

What SEER rating do I need?

This is actually a pretty common mix-up. SEER—or “Seasonal Energy-Efficiency Ratio”—isn’t a measurement of an air conditioner’s size, but its efficiency. Larger or smaller air conditioners don’t necessarily have a higher or lower SEER rating.

The higher the SEER rating, the more efficiently an air conditioner operates. Most air conditioners sold today are at least 13 or—here in California—14 SEER. In our state, that minimum will be 15 SEER by 2023. This makes them much more efficient than air conditioners manufactured decades ago, but not quite as efficient as a 19 or 20 SEER system. Even those systems, in turn, are not as efficient as Carrier’s Infinity® 26 Air Conditioner, which can reach up to 26 SEER!

Here’s another way to look at it: while your air conditioner’s size is determined by your home’s cooling needs—and is therefore fixed—its SEER rating is where you can compare between different systems and make important decisions for your home. Higher SEER systems often cost more upfront, but can end up paying for the difference in costs, and then some, through lower electric bills in the months and years ahead.

How to find the right-sized air conditioner

If you’re familiar with the fairy tale of Goldilocks, you’ll remember that the main character was searching for the right fit—not too large, not too small, but just right. In a similar way, your focus should be on finding the right-sized air conditioner for your home. As discussed above, if you install one that’s too large or too small, you’re going to run into issues.

When your home needs a new air conditioner, you need to work with an experienced and trustworthy HVAC company. The stakes are just too high to entrust your AC installation project to a friend, neighbor, or general handyman.

At Allbritten, we have AC installation specialists on our staff who know just how to find homeowners the perfectly sized air conditioner for their homes. Click the button and fill out the form to set up your free in-home estimate.