What Is the Difference Between a Joule and a Watt?

Energy and power. Though they sound like the same thing, they serve two distinct purposes. The same, then, can be said for the units that define them: a joule and a watt. What actually sets the two apart, though?

Looking beyond the terminology, what is the difference between a joule and a watt? And what would happen if you put the two head-to-head in a matchup? The answer is not so clear-cut. To understand the nuances between a joule and a watt, we’ll first need to make a full comparison between the two.

Joule vs Watt: Side-by-Side Comparison

5 Must-Know Facts About Energy and Power

1. Energy and power cover similar territory, but are ultimately distinct. Think of it like this: energy is defined as the ability to cause change, whereas power is defined by the rate at which energy moves.
2. On average in America, a home will use around 11,000 kWh (or kilowatt hours) of electricity in a single year. Globally, energy use continues to rise by 1-2% annually.
3. Electricity travels at nearly the speed of light, which pans out to more than 186 thousand miles a second. That’s more than 670 million miles an hour.
4. The sun sends more than 430 quintillion joules of energy toward Earth every single hour. Coincidentally, just one hour of the sun’s energy could power the planet for the whole year.
5. 40% of the world’s energy is reserved for generating electricity. This electricity can be sourced anywhere from hydroelectric, nuclear, or solar power (and beyond).

Joule vs Watt: What’s the Difference?

Examining the side-by-side comparison above, it’s apparent that a joule and a watt are pretty different. Just how different, though? We can see they use different symbols, and it’s clear that they have different equivalents — especially in terms of human power.

But what really sets the joule and the watt apart? Let’s break down each SI unit below, taking an in-depth look at the two in order to better distinguish between them.

Joule Explained

For those inexperienced in the field of electrical engineering, a joule and a watt are easy to get confused. After all, looking at the chart above, a joule equals 1 watt a second, while a watt equals 1 joule a second. How could they not be virtually one and the same?

As it turns out, though, the joule vs watt debate is not so cut and dry. In truth, the two are used to measure two completely different things. Put in basic terms, the purpose of the joule is to measure the amount of work it takes for a force to move a mass in the direction of the force being applied.

A joule can also be used to gauge how much energy is converted to heat after an electric current (or ampere) is subjected to resistance (or ohm). A simple formula for calculating the number of joules being exerted is to multiply the work needed to move one coulomb (C) of electric charge through one volt of electrical pressure (or electrical potential difference).

In other words, the number of coulombs (C) times the number of volts (V) equals the number of joules (J). You can also calculate joules by looking at how much work it takes to produce one watt of power for one second.

The joule gets its name from James Prescott Joule. Known best as an English mathematician and a physicist (but moonlighting as a brewer, as well), James Prescott Joule has become a legend of the field for his work with heat, friction, and the connection these things have to mechanical work.

His work is largely credited with leading to the establishment of the first law of thermodynamics: a system’s total energy will always remain constant, even if the energy physically changes from form to form.

Watt Explained

Here’s the basic difference between a joule and a watt: a joule is a unit of energy, whereas a watt is a unit of power. In other words, while the joule is used to measure the amount of work being done, a watt is used to measure the rate energy is transferred.

Based on the International System of Units (SI), one watt is equivalent not to one joule, but to one joule a second (or j/s). Five watts are equivalent to five joules per second, a dozen watts would equal 12 joules per second, one kilowatt — or 1,000 watts — would be equivalent to 1,000 joules per second, and the list goes on.

The name for this particular unit of power derives its name from a Scottish engineer, inventor, and chemist named James Watt. Electrical engineer C. William Siemens proposed Watt as the unit’s namesake in 1882, selecting the man as the next physicist to get a unit named after them.

(Previously, several other leading industry physicists had had their last names used for units.) It would take another 16 years for the unit name to be officially standardized, but when the time finally came, it was embraced enthusiastically.

Looking at the math, it’s actually pretty simple. One watt of power is generated when an object’s velocity is sustained at a constant rate of a meter per second against a constant opposing force sustained at one newton. That’s practically speaking.

When you look at it within the framework of physics, however, it can be broken down into an even simpler equation. One ampere of electrical current (i.e. one amp, or 1 A) with an electric pressure (or electrical potential difference) of one volt (or 1 V) equals one watt of electrical work. Simply put, a watt (W) is voltage (V) times amperage (A).

Joule vs Watt: Real-World Examples

Now, it’s not enough to simply understand a joule and a watt on a mathematical or theoretical level. It’s better to firmly grasp them based on their implications and uses in our daily lives.

Where are you most likely to encounter a joule in the real world? What about a watt? And what do these SI units have to do with us, anyway? Let’s break down some real-world examples of the joule vs watt below.

Joule Examples

1. It takes 60 joules of electricity to run a 60 W lightbulb for one minute.
2. To raise the temperature of a quarter teaspoon of water from 0 degrees Celsius to 1 degree Celsius, it would take one joule.
3. Some refer to joules as “Newton meters.” This comes from Sir Isaac Newton’s famous experiment with falling apples. Lifting an apple one meter and dropping an apple one meter will both release one joule of energy.
4. The human body at rest puts off 60 joules of heat every second.
5. Four joules are equivalent to one gram of calories, approximately. In other words, 100 grams of calories is equivalent to around 418 joules.

Watt Examples

1. Watts can be considered horsepower’s electrical equivalent. In other words, 10 horsepower is equivalent to 7.45 kW (or 7,450 watts).
2. Depending on your particular washing machine brand, washing a load of laundry will use anywhere from 350 to 500 watts. Drying those close in the dryer will need between 1,800 and 5,000 watts, however.
3. A 100 W lightbulb left on for two hours will require the same number of watts used to power a 40 W lightbulb for five hours.
4. Twenty 50-watt bulbs left on for an hour would use 1,000 watts, or one kWh, of power.
5. A human body at rest will only exert around 100 watts of power. A human sprinting at full speed, by comparison, will generate more than 2 kW (or 2,000 watts) of power.

Joule vs Watt: Which One Is Best For You?

So, as we’ve learned, a joule and a watt serve two similar but ultimately distinct purposes. One is used to measure energy, while the other is used to measure power. Additionally, we now know that energy is just power multiplied by time.

Likewise, power is energy divided by time. With that being said, which unit is better for what purpose? Based on what we know, a joule is the preferred unit for measuring energy, while a watt is the preferred unit for measuring power. At the end of the day, they both need each other in order for the math to work.

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