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.