Units 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 names 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 = 1055 Joules (J)|
|Calorie or small calorie (calorie)||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 = 1000 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.
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 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.
The presentation below shows the energy used by various countries or processes on a logarithmic scale as measured in BTUs.
Ok. What we are looking at here is energy scale. Energy scale basically gives us different things. Or it puts different energy consumptions in perspective. For example, a BTU is defined as energy required to raise the temperature of one pound of water through one degree Fahrenheit. This is a small amount of energy. It is one single BTU. That is the unit. It is like a dollar. A dollar can buy you certain things. And when somebody says I bought 10 pencils immediately it strikes you that it is roughly about a dollar or so. If somebody says “I bought a computer” you immediately think about thousands of dollars; a thousand or two, and so on. If somebody says “I bought a car today” which means 10’s of thousands of dollars. Similarly, somebody buys a home. It is 100’s of thousands of dollars. It could be one hundred thousand, it could be two hundred thousand, it could be three hundred thousand. And so on. That is the scale that we are looking at here.
We know on the energy scale 1 BTU is the basic unit. On the same scale, if you look at average daily human food intake is about 10,000 BTUs and in a gallon of gasoline we have about 100,000 BTUs or even slightly higher. What that tells you is that a gallon of gasoline can basically provide a human being with 10 days' worth of supply of food intake or the same number of BTUs basically, or calories in other words. So, technically we can survive with one gallon of gasoline or the equivalent number of BTUs for about 10 days - a human being. Similarly, everyone in the world consumes about 65 million BTUs annually. World per capita annual energy consumption is roughly about 65 million BTUs per person so every person is consuming 65 million BTUs a year.
If you compare that with the United States' per capita consumption that is about 300 million BTUs or slightly over that actually, per year per person. Which means that in the United States, every one of us is consuming roughly 5 times than what an average person in the world would consume - energy wise. And on the same scale, we can compare different things.
This is total annual energy consumption by the United States, as a country, the whole country, which is roughly about 100 Quadrillion BTUs. A Quadrillion BTUs is 10 to the 15. A quadrillion BTUs is 10 raised to the 15 which is a thousand trillion.
Similarly, if you look at here, world per capita energy consumption, that is about roughly 400 Quadrillion BTUs, which is 4 times the United States' energy consumption. To put things in perspective, what we can say is that the United States consumes about ¼ of the entire world's energy. And we say we are consuming a lot of energy and the world is consuming a lot of energy, but look at this here. Look at this. Annual energy that is reaching from the sun, free of cost is roughly 10 to the 22 which is kind of 22 to 23 thousand times more than what the entire world consumes today in a year.
Then, I want you to think about this. Why are we worried about energy? If we are getting 22 thousand times more than what we consume, free of cost, every year, then why are we worried about energy? Think about that.
It is interesting to note that the earth receives more energy in a few hours than all of humanity uses per year. Then one might ask, “Why should anybody worry about energy shortage?" Whether energy is there or not is actually a secondary concern. The real issue is the form in which the energy is available and whether it can be easily converted to the form that we need.