PrintUnits of Measurement
How is energy measured? It is measured in various units by various industries or countries, in much the same way as the value of goods is expressed in Dollars in the U.S. and Yen in Japan and Pounds in Britain.
The table below identifies different units for measuring energy. A lot of it also has some historical context. Our early studies of energy involved heating things up, so we name units based on how hard it was to heat things. Makes sense, right? Now we pass electrical energy to operate many devices, so now we use units that "better" capture this process.
| Unit | Definition | Used In | Equivalent to |
|---|---|---|---|
| British Thermal Unit BTU | A unit of energy equal to the amount of energy needed to raise the temperature of one pound of water by one degree Fahrenheit. Equivalent to energy found in the tip of a match stick. | Heating and Cooling industries | 1 BTU = 1,055 Joules (J) |
| calorie or small calorie (cal) | The amount of energy needed to raise the temperature of one gram of water by one degree Celsius. | Science and Engineering | 1 calorie = 0.003969 BTUs |
| Food Calorie, Kilocalorie or large calorie (Cal, kcal, Calorie) | The amount of energy needed to raise the temperature of one kilogram of water one degree Celsius. The food calorie is often used when measuring the energy content of food. | Nutrition | 1 Cal = 1,000 cal, 4,187 J or 3.969 BTUs |
| Joule (J) | It is a smaller quantity of energy than calorie and much smaller than a BTU. | Science and Engineering | 1 Joule = 0.2388 calories and 0.0009481 BTUs |
| Kilowatt Hour (kWh) | An amount of energy from the steady production or consumption of one kilowatt of power for a period of one hour. | Electrical fields | 1 kWh = 3,413 BTUs or 3,600,000 J |
| Therm | A unit describing the energy contained in natural gas. | Home heating appliances | 1 therm = 100,000 BTUs |
Did You Know?
When writing BTUs, one uses a base of “10” raised to a particular exponent.
For example:
More specific notation involves the following:
To express measurements greater than those with a base of 10, you would do the following:
Here is a fun way to understand your energy use
Prof. Bruce Logan of Penn State published a fascinating way to view your energy and climate impact. Using what you learned in this section, you can start to piece together just how much energy each of us uses to maintain our busy lifestyles.
The premise of this approach is to define (another!) unit of energy, but one with a bit more meaning. The daily energy unit, D. We are all supposed to eat about 2000 food Calories a day to survive. So, let’s set this amount of energy to equal 1 D. Now, how many Ds does the typical U.S. home each day (normally in KWh) or operate a car (normally joules or BTUs )? This method of comparing energy consumption allows us to better understand the scale of our energy habits (which might be shocking!) and tell you how many big mac-powered humans it would take to do what your car does…
Here are a few examples he shows to give you an idea.
- Food for 1 day = 1 D
- Running a single 100 W light bulb all day = 1.03 D
- Average daily electricity use for a US house = 13 D
- 1 gallon of gasoline used in an average car (goes 18 miles) = 15.2 D
- Natural gas for daily heating a US house = 31 D
Once we tally up all the energy it takes to fuel our lifestyle (professional + personal uses), each person consumed about 101 D of energy! (remember this is daily) For comparison, a Swiss citizen consumes about 54 D. Check out his website for more comparisons. Watch this video (5 min 46 sec)
Energy Literacy The daily energy unit "D"
By Bruce Logan Penn State University.
If we want to communicate we need to speak the same language. How can we do that, well we translate it into something we know, into one common language. If you want to communicate when it comes to energy, how do we do that. We translate it into something that we know. Something we all know is that we need about 2,000 calories a day to live. That's something we can relate to. But how do we relate that to other things that either consume energy or produce energy in our lives.
Well, for example, if I were to take that 2,000 calories a day that I eat and use that energy to power 100 watt light bulbs, how many could I power. 3 4 5 10? The answer is just one light bulb. A single light bulb running continuously consumes the equivalent of about 2,000 calories a day.
Now if you look at all the things that we encounter in our lives in terms of energy units, so many of them have different units. For example 2,000 calories with a capital C is really 2,000 kilocalories. Daily food for a horse is 20,000 kilocalories. The energy in a gallon of gasoline 114 thousand BTUs or British thermal units. Or maybe you look at the engine in your car and it's 120 horsepower.
How can we understand these units. Well, one way is we could put them all on the basis of a kilowatt hour. However, look at those numbers. They're all still rather confusing and it's tough to relate to a kilowatt hour. So we need to find something that's not quite so confusing.
I propose that we define 2,000 calories a day, or the food that one person eats, as the unit 1D. One daily energy unit. If we use that unit, we could say well, our home uses 13D. Or while we eat 1D, a horse needs to eat 10D. The energy in a gallon of gasoline is about 14D.
So this common unit of D, allows us to now compare all these things in terms of something that we know, that is how much food we need to stay alive. And that unit of 1D is the same to everybody on the planet. Some people need a little bit more some people need a little bit less, but the general concept of the food we eat every day can help define what energy consumption is like.
So when you look at things now, for example, you can see the electricity that we consume per capita, not necessarily just in your home, but averaged across to all people in the US, is about 40 D, and all energy normalized per person in the U.S. is about 104 D. That makes this unit of D very nice because it ranges for the food and other activities up to about a maximum of about a hundred.
Look at your car which say gets 18 miles per gallon and your commuting 18 miles. Your commute costs you about 14D, 1 gallon of gas. If you were to travel in a more fuel-efficient car, say a Toyota Prius or something like that, that commute might only take 4.9D because you would use less gasoline. Or what if you used an electric car. Well to charge that car and use the energy in that battery, it's about 27 kilowatt hours to go 100 miles or 2.1D.
Let's say that you want to put solar panels on your house. Well each solar panel produces 0.43D. That means averaged over the day and with typical sunlight, and for example Pennsylvania, you would achieve about 0.43 D of energy capture out of each one of those panels. So if your house uses 13D, well you need about 30 solar panels. If your car is an electric car and it uses 2.1 D, then your commute needs about 2.1D or about 5 or 6 solar panels.
How much energy should we use. Well there's a study done by ETH Zurich which suggests a 2,000 watt Society. 2,000 watts is about 20D. Currently, Switzerland has the average population having about 53 D which is about half of what we consume in the U.S., which is about 104D. So if we're gonna have to try and reduce our energy consumption that's gonna have to go down quite a lot.
Going forward, how can we do that. Well, can you reduce the energy for your commute. Can you save more energy at home or energy you say for transport or food or put a solar panels on your house and reduce your consumed D.
So the question really is, going forward, how can we make that 20D green.
Thanks for listening and I hope that you find a way to use D in your life to understand the energy you can see.