When looking at energy use in everyday life situations, it is easy to overlook what the units used actually mean. When getting the electric bill in the mail, most people will simply compare the kilowatt-hours from last month to this month and note if their bill has gone up or down. When buying a new energy-efficient dryer, you know the fewer watts used the less energy it will be. The same mental comparisons are used all the time by people who do not have to deal with energy extensively– such as with the horsepower of a car or the calories in a sandwich.

However, it is all too common for people to forget the real significance of and differences between various units of measure related to energy and power use once they pass their high school physics class. Newscasters will constantly use kilowatts and kilowatt-hours as if they’re interchangeable (they’re not). Writers will misrepresent statistics online as if the difference between megawatts and gigawatts are not massive (they are).

For those of us that work in the energy industry, these numbers are much more tangible and easy to understand. However that does not describe a majority of citizens who are having these statistics thrown at them all the time, so this article will serve as a reference and allow you to re-up your energy statistics literacy.

The Basics

Energy vs. Power

The cardinal sin when dealing with energy units is confusing energy and power, a mistake that is unfortunately one of the most common as well. Even in mainstream news articles, it is not uncommon to see the total energy used for something to be listed in watts or vice versa (e.g., this article quotes the rate of energy use of a soccer stadium in kilowatts per hour, which you will shortly understand to be nonsensical if taken literally). So clearing up the confusion here is top priority.

The technical definitions of energy and power, according to the Energy Information Administration (EIA), are as follows:

Energy: The capacity for doing work as measured by the capability of doing work (potential energy) or the conversion of this capability to motion (kinetic energy)

Power: The rate of producing, transferring, or using energy, most commonly associated with electricity

Put simply, energy is the total work that is done while power is the rate at which that work is done. This concept can still be a bit tricky, so the easiest way to keep it straight is through metaphors. As one example, you can think of the relationship between energy and power as water flowing from a hose to a bucket. The volume of water that has been added to the bucket at any given point is comparable to the total energy use, while the rate that the water is flowing from the hose into the bucket can be considered the power. Another useful metaphor is to consider power to be the speed a car travels along a highway, while the total distance traveled would be the total energy. The main point is to think of power as a rate that is occurring with time (gallons of water per second, miles per hour) while the energy can then be thought of as that rate multiplied by the amount of time to get the total quantity (gallons of water per second times total seconds = total gallons of water, miles per hour times total hours = total miles driven).

To bring it to real world applications of energy and power, think of a light bulb in the lamp of your living room. The light bulb might be rated at 60 Watts, which is the power rating. 60 Watts is the rate of energy use of the bulb, and if you leave it operating for 2 hours then the total energy use is 60 Watts times 2 hours or 120 Watt-hours. Watt-hours, often divided by 1,000 to be expressed in kilowatt-hours, are the total energy use you will see come up on your monthly power bill (for more real-world applications of power and energy calculations, see the recent blog post on the energy used in various Thanksgiving turkey cooking methods).

Once you understand the difference between energy and power, you will start to see them used improperly all too often.

SI units vs. Imperial units vs. every other type of unit

To anyone who has to deal with the variety of units available to measure the same quantity, it can seem very confusing and unnecessary. Certainly it would be easier if everything and everyone used the same units and no conversion was needed. Unfortunately, that is not the world we live in for a variety of reasons– everyone has seen or heard how hard it has been to try to get the metric system adopted in the United States.

The reality is that there are many different units because these units originated at different times, by different people/industries, for different uses. The development of the metric system during the French Revolution was the first attempt to create internationally agreed upon units. Prior to that time, the world was a much larger place and it was not uncommon for units that even carried the same name to vary in actual measurement depending on where you were and who you asked. As science and trade expanded with the ever-shrinking global stage, units became more and more standardized until the International System of Units (SI) was created in the mid-20th century. These units are standard and widely accepted across the scientific landscape, no small victory for unit standardization.

Even with that success, however, many industries were already set in their way. For example, even though the automotive industry could use the widely accepted wattage to describe the power of an engine, people already understood horsepower in the context of a car. Because of the inertia and history of units like this, the implementation of the SI system did not take off in all sectors. While this may have been the easiest choice for those industries, it leaves the layperson with an alphabet soup of units and abbreviations to wrap their head around. Hopefully this article will do a small part to clearing that all up.

Prefixes

Another important part of the tangled web of units, particularly among SI and metric units, is the use of standard prefixes. Prefixes are used to take a standard unit and modify it by a power of ten. A familiar example would be the difference between a meter and a kilometer. Kilo- is the standard prefix for a multiplier of 10^3 or 1,000, which is why a kilometer equals 1,000 meters. These types of prefixes, summarized in the table below, can be applied across all sorts of units and the meaning is always the same– look at the power of ten multiplier and apply it to the unit.

The prefixes at the extreme of either end (such as yotta- and yocto-) are rarely used because they are so large/small that they are not needed to describe real, tangible energy/power quantities you’ll come across. The ones that are commonly used include giga-, mega-, kilo-, milli-, and micro-, and in fact some of the units described in the below tables will have those prefixes because the power-of-ten-adjusted units are more commonly used in certain applications than their base units.

Units to know

Energy

With all that background out of the way, we can look at 24 various units used to measure energy. Some of these are more common and will be familiar to most people, others are more niche and relate to specific industries or fields of study, while others still are rarely used but are still interesting to consider. Again keep in mind you may run across more units made up of the measures below combined with one of the prefixes above– simply use the prefix multiplier to modify the designated unit in the below table.

This first table will list these energy-measuring units, from smallest to largest, along with the manner in which they are typically used, the qualitative fundamental equivalence by definition, and the standard quantitative reference.

Table 2: Units of Energy Across Industries and Applications

UNIT ABBREVIATION TYPICAL USE FUNDAMENTAL EQUIVALENCE STANDARD REFERENCE

*These values are approximate because different grades of oil/gas have slightly different energy equivalents, and thus different agencies/bodies sometimes use slightly different measures of them.

**It’s important to note the difference between calories and Calories– Calories with a capital C are the nutrtional Calories everyone is familiar with counting on diets. These Calories are actually known as kilocalories and are 1000 thermonuclear calories, so do not mix up Calories and calories…

To make some more sense of this array of units, both massively large and incomprehensibly small, the following table puts the units into some more context. In this table, you’ll find a real-world example of what can be done with a single unit of that energy measurement, how many Joules it equates to for comparison’s sake, and the multiplier needed to get from the previous unit of energy to that one.

Power

The same exercise can be done for units of power (or rate of energy over time), as there are just as many different units for various industries, applications, and technical necessities. For power, we’ll focus on 17 of the more commonly used units– though remember you might come across all of them modified by the previously discussed prefixes.

Again, this first table will list all the power-measuring units, from smallest to largest, along with the manner in which they are typically used, the qualitative fundamental equivalence by definition, and the standard quantitative reference.

Table 4: Units of Power Across Industries and Applications

UNIT ABBREVIATION TYPICAL USE FUNDAMENTAL EQUIVALENCE STANDARD REFERENCE

Again, a useful way to make sense of all these power units is to give them more meaningful context. The next table shows some of the real world examples of these different levels of power output, converts them all to Watts for the sake of comparison, and the multiplier between two consecutive units.

Conclusion

Armed with the knowledge of these units of energy and power, you’ll be well prepared to tackle statistics anew– you’ll have useful context for how much energy was in the recent 5,000 barrel oil spill on the Keystone Pipeline (using the above information, we can calculate that 5,000 barrels of oil is over 30,000 Gigajoules– or equivalent to the average annual electricity consumption of over 700 American households), or you’ll also have not so useful (but fun!) context for the energy content of a gallon of gasoline (the same as over 127 slices of large cheese pizza or 30 kg of TNT). Either way, being literate in your scientific and energy-related units will make you a more informed consumer of the news– if only everyone editing the news could do the same and stop using ‘Watts per hour’!

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For examples of some fun ways these units of energy conversion can be applied to real-world circumstances, check out blog posts on energy in slapshots, energy and power use in the Star Wars universe, and the energy use in New Years fireworks.