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BTU: What Does It Mean For You and Your Air Conditioner?

As you start shopping for a new air conditioner, chances are you’ll come across the acronym BTU. And that means what, exactly?

BTU is short for British Thermal Unit, a unit of measurement that shows just how much energy your air conditioner uses to remove heat from your home within an hour. It may seem overly technical, but BTU is an important metric that can help you determine the kind of air conditioner you need for a home your size.

Getting an air conditioner without paying attention to its BTU rating could seriously crimp your comfort if your system doesn’t have enough energy to cool your home. Follow along for a breakdown of BTU, so you’ll know what to look for as you browse our inventory of air conditioners.

Why BTUs should be important to you

If you want to understand BTU, it helps to understand how air conditioners work. It’s OK if you don’t spend each day thinking about the inner workings of air conditioning systems and how they seem to work so well at making your rooms feel cool and comfortable. That’s why we created our HVAC 101 page to offer a more detailed explanation of how air conditioners function.

For now, though, just know that air conditioners remove heat from the air inside your home and move it outside. Using an intricate system of coils and fans, they replace that hot and humid air with cooler air that travels throughout your home via ductwork. Think of it like tubes in a water slide. You get in, slide down the tube and splash into a really big pool. Your air works the same way. The air enters the tube, slides through and then works its way throughout your home.

When we talk about air conditioners removing heat from your home, it’s the same as saying that they’re moving energy. BTU tells you how much energy your air conditioner is using to do all that. So, if you see that your air conditioner has 12,000 BTUs, that means it’s absorbing 12,000 British Thermal Units of heat each hour and moving it outside so your environment inside feels better.

But why measure with BTU?

BTU doesn’t just revolve around your air conditioner. It’s used across the globe to measure the amount of heat that’s needed to raise or lower one pound of water by one degree Fahrenheit. That means it can be used to rate energy transfer for furnaces, heat pumps, cooking devices and other heating and cooling appliances.

While air conditioners don’t heat your home, they do specialize in removing unwanted heat (which you can also think of as energy) from it. HVAC professionals use BTUs to measure both heat loss and heat gain, which is why we apply that metric to your air conditioner’s performance.

But you don’t need to be an HVAC manufacturer or technician to leverage BTUs for your space. Understanding what the rating means is especially useful if you want to avoid the drawbacks of buying an air conditioner that doesn’t provide the exact kind of comfort you want. Here’s what we mean:

Getting an air conditioner with too many BTUs for a home your size can raise your energy costs. That’s because your air conditioner will run for a shorter period of time and waste more energy doing what it’s built to do. An air conditioner with BTUs that exceed the recommended square footage in your home may also take on more wear and tear over time because it will initiate its on/off cycle more frequently. It could also create a subpar cooling experience because your AC’s compressor may turn off much sooner than it should. So, instead of your AC removing an adequate amount of heat from your home, it leaves some behind, creating a hot, sticky, humid environment you probably won’t enjoy.

Getting an air conditioner with too few BTUs for a home your size isn’t much better. Your air conditioner may never stop running because it’s trying to reach a temperature in a large space that it just doesn’t have the capacity to cool. It may never sufficiently cool all your home, creating uneven temperatures across your house. And it could cost you more in energy bills because your AC’s expending extra energy to do what you tell it to do via your thermostat.

Knowing the right BTUs for a home or dwelling your size is essential if you want to avoid these issues.

Choosing the right BTU for your air conditioner (and your home)

Generally speaking, an air conditioner needs about 20 BTUs for each square foot of living space that it’s cooling, according to ConsumerReports. To get an approximate idea for how many BTUs you need, multiply the square footage of your space (whether that’s a single room or your entire home) by 20.

For example, let’s say you need to find an air conditioner kicking out enough energy to cool your 1,400-square-foot home. You should look for a unit that’s about 28,000 BTU. Remember, this is just a simple, general estimate that doesn’t apply to all situations. A true BTU calculation will take your ceiling height and the size of your doorways and windows into consideration, as well.

This BTU chart gives you a rough guide to follow based on square footage.

House Square Footage BTUs Needed
100-150 5,000
150-250 6,000
250-300 7,000
300-350 8,000
350-400 9,000
400-450 10,000
450-500 12,000
500-700 14,000
700-1,000 18,000
1,000-1,200 21,000
1,200-1,400 23,000
1,400-1,500 24,000
1,500-2,000 30,000
2,000-2,500 34,000

BTUs are also part of the Manual J Calculation, a mathematical method HVAC professionals use to accurately determine how much heating and cooling a home your size requires. You can request a Manual J calculation from an experienced HVAC technician.

How BTUs and tonnage are linked

We briefly mentioned cooling capacity earlier, but we need to get more specific about what that means in terms of your BTU. Once you’ve got an idea of how many BTUs you may need for an AC unit, you’ll want to know the corresponding tonnage for your air conditioner.

Tonnage is not a measurement of your air conditioner's weight. In HVAC terms, it’s another way to describe your unit’s cooling capacity, or how much heat your AC is capable of absorbing in order to reduce a room’s temperature. One ton of cooling (also called refrigeration) refers to the amount of heat needed to melt a pound of ice over 24 hours, according to FurnaceCompare.com. It’s roughly the same as 12,000 BTUs per hour. That means a 2-ton AC unit has a 24,000 BTU/hr cooling capacity because 12,000 x 2 = 24,000. A 2.5-ton unit has a 30,000 BTU, a 3-ton unit has a 36,000 BTU and so on.

Wait, why ice? Good question! Before the advent of electric air conditioning, people cooled their homes and businesses with large blocks (or tons) of ice. As it slowly melted, the ice absorbed the heat in the room. The process worked just like the modern-day air conditioner. The ice didn’t produce the cool air, but instead removed hot air from the room to make it cooler and more comfortable. HVAC professionals continued to use “tons” of ice to measure how much heat air conditioners remove even as the ice method became obsolete in the face of new, contemporary air conditioning methods.

The takeaway: Merge what you know about BTUs and tonnage to determine the best-sized air conditioner and cooling capacity for your home.

Calculating kilowatts using BTU

Before you commit to a particular AC unit, you may want to know how much electricity it may consume to cool your home. That way, you get an idea of how it will influence your utility costs. BTUs, when converted to kilowatts and kilowatt-hours, can help you figure that out.

First, a quick explainer: A kilowatt is 1,000 watts, which is a measurement of power. A kilowatt hour measures the amount of energy needed to run an appliance, tool or machine for one hour. When it comes to your air conditioning, a kilowatt hour tells you how much energy your AC is using, which could be instrumental in helping you decide what kind of unit to buy.

To convert BTUs to kilowatts, take your BTUs and multiply them by 0.000293 — the amount of kilowatt-hours in one BTU. For example, if you’re looking for an air conditioner with 18,000 BTU, you’d get 5.27 kilowatts, or 5,270 watts. Let’s say your AC runs for four hours a day. Within that period, your air conditioner would use 21.08 kilowatt hours (5.27 kilowatts x 4 hours).

18,000 BTU x 0.000293 = 5.27 kilowatts

5.27 kilowatts x 4 hours a day = 21.08 kilowatt hours for your air conditioner

If you want to determine your estimated energy costs, multiply your kilowatt hours (21.08, in this case) by 30 for 30 days in a month. That gives you 632 kilowatt hours per month. You’ll need to multiply this figure by the electricity rate you’re charged each month. As of May 2021, the average electricity rate in the U.S. was 13.71 cents per kilowatt hour, according to the U.S. Energy Information Administration. By multiplying your electricity rate by your kilowatt-hours, you can determine your energy costs:

21.08 kilowatts x 30 days in a month = 632 kilowatt-hours per month

632 kilowatt-hours x 0.13 cents per kilowatt hour = $86.70

We know that’s a lot of number-crunching. But hopefully those calculations can help you make a more informed purchasing decision.

So, what does all this mean for your AC?

BTUs are an essential part of identifying the air conditioner that will best fit your needs. Ultimately, your goal should be to find an energy-efficient system that adequately cools your space while helping you save on energy costs. So, as you continue shopping, be sure to check out the BTU rating and do some quick calculations to figure out if it makes sense for you.

If you need help, get in touch with your local Trane dealer who can help you understand the kind of system that’s right for your home.

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