What is a British Thermal Unit (BTU)?

April 1st, 2020

Dear Readers,

Through William and I’s endeavors to understand Passive House standards, and my personal endeavors to gain a basic understanding of architecture, I have come across quite a few significant moments. One of which being the attempt to understand Passive House criteria set by the Passive House Institute (PHI- the primarily German one, not to be confused with PHIUS- the primarily United States one).

As I meandered my way through PHI standards, I realized that I was learning absolutely nothing.

At all.

And please do not think that I am saying it is PHI’s fault! I entirely accept the responsibility of my own shortcomings in understanding technical terminology. I have, however, come to the realization that terms and their definitions can be created from one person to the next, and if you have enough individuals nodding their heads and smiling at you… then that makes those terms and definitions legitimate.

In my attempts to write a blog about PHI criteria, I realized that I kept typing the letters ‘BTU’ without knowing what they really meant. But my sentences sounded well informed, even though I didn’t really know what I was talking about. So, I stopped. The more I wrote, the more questions I had. And then I continued to read and research. Also resulting in more questions.

Therefore, dear readers, I am taking a step backwards, and I am going to try to explain, and understand for myself, what those three letters mean.

Are the units actually British?

From the sounds of it, yes. A British man made up the term and enough people nodded and smiled at him, therefore making British Thermal Units a real form of measurement.¹

What do they measure?

BTUs are a measure of heat content. According to the U.S. Energy Information Administration (EIA), a BTU “is the quantity of heat required to raise the temperature of one pound of liquid water by 1 degree Fahrenheit at the temperature that water has its greatest density (approximately 39 degrees Fahrenheit).”² Which, sounds like a bunch of gibberish, because I cannot create a quantified image of a pound of water at its greatest density in my head… But! The U.S. EIA does give a great comparison to a match: One BTU is roughly the amount of heat exerted by a single burning match.

You can throw BTUs at a lot of things to measure how much heat they are putting out…

  • You can use it to measure how much heat is being exerted by your wood stove.
  • How much heat one gallon of propane will provide.
  • How much heat a mini split unit can put out (we heard ‘BTU’s tossed around a lot when my parents were deciding whether to have a mini split system put into their home.)

BTUs also become super helpful when fancy equations for things like heat loss coefficients (HLC) come into play. Heat loss coefficients are used to find out how much heat is escaping from your home. Because you are measuring how much heat is traveling through those low R-value walls, or leaking through those gaps in your doors, or even escaping through your ventilation systems to find your HLCs, you are essentially measuring how many BTUs your home is wasting.

For example, my family’s home loses about 7,000 BTUs per hour from interior air conducting to the exterior through our walls and windows. Our home loses another 3,000-ish BTUs per hour through leaks in our door seals. That is a rounded total of 10,000 BTU/hr we are losing from our home. Which is an equivalent amount of heat to 10,000-ish burning matches.

That is why, when mini split people come around to estimate what it would take to install a mini split system into your home, they want to know the R-value of your walls and ceiling and take a look at those leaky doors and windows. Then, they estimate how many BTUs in total your system would have to put out in order to compensate for all the escapee BTUs that will be running away to the great yonder.

For example, in order to heat my family’s 3,000-ish square foot home, it was roughly, and overly, estimated that it would take 66,000 BTUs to heat our home. That is 66,000 burning matches…

There you have it, dear readers! BTUs.

Thank you for reading! And, perhaps, thank you also for your nods and smiles… you make us feel legitimate 😉


Shelby Aldrich

1.  Holohan, Dan. “The Origin of the British thermal unit,” 11 Jan. 2016. HeatingHelp.com. https://heatinghelp.com/blog/the-origin-of-the-british-thermal-unit/. Accessed on 2 Feb. 2020.

2. U.S. Energy Information Administration. “Units and calculators explained-British thermal units (Btu),” 12 June 2019. https://www.eia.gov/energyexplained/units-and-calculators/british-thermal-units.php. Accessed on 2 Feb. 2020.


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