Breaking Down BTUs and Why They Matter for AC Performance

D.P. Taylor
Written by D.P. Taylor
Updated March 9, 2022
An AC system in the backyard of a house
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  • BTUs measure the energy output of your AC system.

  • It takes about 20 to 35 BTUs to cool each square foot of space.

  • Higher BTUs cool a room faster, but boost humidity and wear your unit down.

  • EERs measure the energy efficiency of an AC unit.

  • It costs $0.10 to $0.20 per BTU to have an AC unit installed.

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One of the key attributes of an air conditioning system is BTUs. While you know that BTUs have to do with power, it may not be clear exactly what the term refers to, and why it is relevant to the system you choose.

This guide breaks down what a BTU is, how many you need depending on the size of the space you want to cool, and how much you can expect to pay per BTU. With this information, you’ll be better able to choose the best AC unit for your home.

What Does BTU Mean?

BTU stands for British Thermal Unit, and its purpose is to serve as a unit of measurement that describes how much energy your air conditioning system is using per hour. More specifically, a BTU measures how much energy a unit needs to cool a pound of water by 1 degree Fahrenheit below the current temperature.

Why BTU Matters

BTU matters because too few BTUs mean your AC unit won't be powerful enough to cool the space, but too many BTUs will wear down your unit faster and make your room humid. As a result, it's important to strike a balance and figure out the right BTUs for your home.

How to Determine The Proper AC Size and BTU

Generally, it takes about 20 to 35 BTUs to cool each square foot of space. For example, 5,000 to 6,000 BTUs is enough to cool 150 square feet, while you'll need 11,000 to 12,000 BTUs to cool 500 square feet. The higher the BTUs and the larger the space, the more efficient the system's cooling efficiency.

Factors That Affect BTU Size

The following factors will have the biggest impact on the number of BTUs you should target:

  • Size of the space you intend to cool

  • Number of windows and floors in the home

  • Insulation and flooring type

  • Number of family members in the home

  • Size of your kitchen and how often you cook in it

Higher BTU Isn’t Always Better

A technician checking an AC system
Photo: Pramote Polyamate / Moment / Getty Images

Generally speaking, a higher BTU is not better for your AC system. That's because higher BTUs will make the room more humid. It will also put more wear and tear on the unit, requiring more maintenance and shortening its lifespan. The key to comfort is finding a balance: an AC armed with enough BTUs to get the job done, but not too many for the space you're cooling.

Will an Air Conditioner With a Higher BTU Cool My Room Faster?

Yes, an air conditioner with higher BTUs will be more powerful and will therefore cool your room faster. But as noted above, this comes with significant drawbacks, like higher humidity and greater wear on the AC system itself (not to mention higher energy bills).

BTUs and EERs, Explained

While the BTUs measure the amount of energy used, the EER notes the unit's energy efficiency, or how well your AC will cool down a room compared to power usage. You can calculate EER by dividing the BTU rating by how many watts the unit uses. It's a helpful measurement if you're trying to compare to AC systems. The higher the EER, the more efficient the system is.

How Much Does an AC System Cost Per BTU?

The cost to install an AC system by BTUs is about $0.10 to $0.20 per BTU, with the price being on the lower end for larger, more powerful systems and on the higher end for smaller systems. 

An AC unit with a capacity of 18,000 BTUs would could about $2,500 to $4,500 to install, while a large 48,000-BTU system would cost $4,200 to $6,200 to install.

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