∫ Useful Energy Units

If you’ve read any news articles about energy you’ve probably seen the word kilowatt-hours, and any number of other equally opaque units of measurement. We looked before at the critical difference between kilowatts and kilowatt-hours. (Quick refresher: kilowatt is a rate of energy use, kilowatt-hour is a measure of total energy used). But how do these units relate to real life? What about BTUs, therms, and Joules? What do those really mean, physically? Is a ton of carbon dioxide different from a ton of cooling? And are we talking about metric tons, long tons or short tons?


Many Flavors of Energy Units

The following all measure energy: kilowatt-hours (kWh), British Thermal Units (BTUs), Therms, Calories (Cal), Joules (J), and horsepower-hours (hph). It may be surprising that we have so many varieties, but energy is fundamental to much of science, technology, and nature and, consequently, our lives. Energy is a measurement of how much work is done. When your lamp is on, that lamp is using electrical energy to produce light. When you lift a shopping basket off the floor, your body uses chemical energy to overcome gravity’s pull on the basket. Everything we do needs energy.

The kilowatt-hour is the most important unit of energy and the standard for electrical energy in buildings. If you are running a 1,000 Watt (or 1 kilowatt) space heater for 1 hour, you use 1 kilowatt-hour of energy.

A British Thermal Unit is a common measurement of heat (which is a form of energy). It is defined based on the specific heat of water. Specific heat is a measure of how much heat is required to change the temperature of a material by one degree. Every material has its own specific heat. For water, the heat needed to raise one pound of water by one degree Fahrenheit is exactly one BTU. An example: To heat 1 pound of water from 70°F to 80°F would require: 10 BTUs. You may see the unit “Therm” on your energy bill. A Therm is defined as 100,000 BTUs; it is also roughly the energy released when 100 cubic feet of natural gas is combusted.

With a capital ‘C’, Calorie means kilocalorie, the same measurement you see on your cereal box. Like the BTU, a Calorie is defined based on the specific heat of water. For water, the heat needed to raise a kilogram of water by one degree Celsius is exactly one Calorie. Notice a pattern? An example: To heat 1 pint of water (0.473 kg) from 20°C to 30°C (68°F to 86°F) would require: 4.7 Calories. About one almond or Jelly Belly.

One-thousand calories with a lowercase ‘c’ combined equal one Calorie with an uppercase ‘C’. If you can find a fun historical reason for why, let us know.

A Joule is most frequently used by scientists. It is defined as 1 kilogram times meters squared per second squared. What is that? The energy required to lift one full soda can one foot high. A thousand Joules is one kilojoule (kJ). If you use one kilojoule of energy per second, that’s equivalent to one kilowatt (kW) of power. So every second it’s on, your space heater is using the same amount of energy that it would take you to lift 1,000 cans of soda one foot high.

Horsepower has many definitions, but for our purposes we’ll consider mechanical horsepower which is equal to 746 Watts (or 0.746 kW). It was initially devised to compare the output of steam engines to a certain large animal.

Those are some – but not all – of the key units of energy. Next, we’ll look at the many flavors of tons.


Fun with Tons

Basics first. While a ton can be many things, the most fundamental technical use of the term is as a measure of mass. Mass is the stuff that makes up the world. The combined mass of all the atoms in your body is likely between 50-100 kilograms. Your laptop has about 2 kilograms of mass. Your phone has about 300 grams of mass, while your car has about 1,500 kilograms of mass. Those are all measures of stuff. In the presence of a gravitational field, mass has weight. By contrast, in outer space, you are weightless (but not massless – that is, you no longer feel gravity but you’re still made of stuff).

A metric ton is the most common measurement of ton. It is equal to 1,000 kilograms or 2,205 pounds. (Bonus points for those who knew that a pound is really a unit of weight not mass. Also, we’re technically being lazy here and should say 1,000 kilograms of mass equals 157.5 stones, if you want to be particularly rigorous).

A long ton or Imperial ton is equal to 2,240 avoirdupois pounds (An avoirdupois pounds is the technical term for regular, 16 ounce pounds. Bonus points for anyone who can pronounce ‘avoirdupois’).

A short ton is equal to 2,000 pounds. This is the standard ‘ton’ in the United States. When someone says “I ate a ton of mac and cheese last night,” if they were using the phrase literally and not figuratively, odds are they mean they ate a short ton of mac and cheese.

A ton of carbon dioxide is slightly different because it usually refers to a metric ton of carbon dioxide gas. It is hard, however, to picture what that is. As a reference point, one ton of CO2 gas would occupy about 150,000 gallons (US gallons) of space. That means just five tons of carbon dioxide could fill an entire Olympic size swimming pool. Every year, the world emits billions of Olympic size swimming pools worth of carbon dioxide.

A ton of cooling or refrigeration ton is actually a cooling rate. It comes in units of energy per time (like kilowatts), and measures how much cooling an air conditioner can provide. A refrigeration ton is defined as 12,000 BTU/hour; the amount of heat a one-ton block of ice removes from a room as it melts over 24 hours.

Fun Ton: sadly, this is not yet an officially recognized unit. But if you’ve ever experienced it (and what better way to do that than learning about energy!) you know how great it can be. Some researchers posit that it is closely correlated but not causally related to Fort Funston in San Francisco.

That’s it on units for now. There are lots more to explore and we’ll come back to them in a future post. Until then, next time you come across an article discussing millions of kilowatt-hours, hopefully you’ll have some appreciation for the incredible amount of energy that represents!



Strange fact: You may have heard the expression “a pint’s a pound the world around.” It refers to one pint of water weighing one pound. While this is a handy phrase, it’s actually only an approximation. Consider that:

1 US fluid ounce of water fills 29.57 milliliters (mL)

1 US pint = 16 fluid ounces = 473.18 mL

An ounce weighs 28.35 grams

1 pound = 16 ounces = 453.59 grams

The density of water is exactly 1 gram/mL.

So 1 pint of water actually weighs 473.18 grams, or 1.04 pounds.

10 Responses to ∫ Useful Energy Units

  1. Great post! This is the best explanation I’ve seen of how to conceptualize a Joule. Also, great write-up regarding tons. I’m still rooting for formal recognition of the “fun ton.”

  2. “1 US pint = 16 fluid ounces = 473.18 mL…..”

    …. and one UK pint (the true home of the pint – of beer at least) is 568ml. So it’s really not a pound the world around.

    …. also the last pint i bought cost around £3.50 unfortunately

    Great webpage nonetheless!

  3. Personally my favorite is kilowatt-hour!! I guess BTUs are pretty alright, but why isn’t there an American Thermal Unit?

  4. I have to say, BTUs are probably my favorite, especially when expressed as a rate, and in large quantities: MBHs rule! Tons are useless; you just end up converting to MBHs anyway.

  5. Angelica says:

    Wow, I just learned more reading this post than in many years of science class. Excellent explanations; very accessible. Also definitely breaking out the Strange Fact at the next party I attend.

  6. you are a gifted explainer! i wish i had remembered to ask you to explain batteries for a case i’m working on. instead, i slogged through hours of research on wikipedia. ai yai yai!

    • We actually have a blog post on batteries upcoming in the next month or so. Once it’s up, feel free to direct your students there and we’ll try to make an engineer available to answer questions.


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