A watt is a rate: one joule per second. The power P tells you how much energy a device converts per second, so P is E divided by t. A 40-watt fan converts 40 joules every second. For an electrical appliance, the power is the voltage times the current: P is U times I.
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A 40-watt fan uses 40 watts per hour, doesn't it? It sounds logical, but it is wrong. A watt is already a rate: how much energy a device converts per second. That is called the power P.
One watt is one joule per second, so P is E divided by t. The fan converts 40 joules every second. "Forty watts per hour" is therefore just as strange as "a hundred kilometres per hour per hour" if you mean the speed of a car. If you want to know how much energy it converts in an hour, you turn the formula around: E is P times t, with the time in seconds. That is 40 times 3600 is 1.4 times ten to the fifth joules.
For an electrical appliance, the power is the voltage times the current. Voltage is energy per charge: one volt is one joule per coulomb. Current is charge per second: one ampere is one coulomb per second. Multiply those two, and the coulomb cancels out and you are left with joules per second. That is a watt: P is U times I.
In the worked example, a laptop charger delivers 19 volts at 3.4 amperes. That is 65 watts, and in an hour it converts 2.3 times ten to the fifth joules.
For upper secondary physics, in electricity.
In this video
The full explanation, in writing
Per hour
0:00A forty-watt fan uses forty watts per hour, doesn't it? How much energy a device converts per second is called the power P. So a watt is not an amount of energy, but a rate: one watt is one joule per second. P is E divided by t. That fan converts forty joules every second.
0:28"Forty watts per hour" is therefore just as strange as "a hundred kilometres per hour per hour" if you mean the speed of a car. If you want to know how much energy it converts in an hour, you turn the formula around. E is P times t, with the time in seconds: forty times three thousand six hundred is one point four times ten to the fifth joules.
Voltage times current
0:53The power of an electrical appliance is the voltage times the current. Voltage is energy per charge: one volt is one joule per coulomb. Current is charge per second: one ampere is one coulomb per second. Multiply those two, and the coulomb cancels out and you are left with joules per second.
1:15That is a watt. So P is U times I. Your turn. A laptop charger delivers nineteen volts at three point four amperes. How many watts is that?
Sixty-five watts
1:30For energy, mind the time in seconds. Nineteen times three point four is sixty-five watts: sixty-five joules per second. How much energy in an hour? An hour is three thousand six hundred seconds, so sixty-five times three thousand six hundred: two point three times ten to the fifth joules.
Three things to remember
1:50Three things. A watt is a rate: one joule per second. The power of an electrical appliance is voltage times current. And energy is power times time, with the time in seconds if you want to end up in joules.
Two questions
2:05Two questions. One. A toastie maker of eight point zero times ten to the second watts is on for four point zero minutes. How many joules does it convert? Two. A USB lamp on five point zero volts has a power of two point five watts. What is the current?
The answers
2:26The first: four point zero minutes is two hundred and forty seconds. Eight hundred times two hundred and forty is one point nine times ten to the fifth joules. The second: I is P divided by U. Two point five divided by five point zero is zero point five zero amperes.
Kilowatt-hour
2:44Want to know why your energy company doesn't charge in joules but in kilowatt-hours? Then watch the video about the kilowatt-hour on my YouTube channel.
Frequently asked questions
What is the difference between a watt and a joule?
A joule is an amount of energy, a watt is a rate: how many joules are converted per second. One watt is one joule per second. If you want the energy, you calculate E is P times t, with the time in seconds.
How do you calculate the power of an electrical appliance?
Multiply the voltage by the current: P is U times I. A laptop charger that delivers 19 volts at 3.4 amperes has a power of 65 watts.
Why is P equal to U times I?
Voltage is joules per coulomb and current is coulombs per second. Multiply them, and the coulomb cancels out and you are left with joules per second, and that is a watt.
Next: why your energy company doesn't charge in joules but in kilowatt-hours, and what a kilowatt-hour is in joules.
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Physics with Thomas: physics explained with pictures where they add something, and without them where they distract.
In this series
Electric current6:12
Voltage and capacity4:43
Connecting ammeter and voltmeter4:10
Resistance and Ohm's law4:21
Resistance of a wire4:40
Variable resistor, NTC, PTC and LDR3:40
Series circuit3:32
Parallel circuit3:56
Series-parallel circuit3:37
Circuits practical test5:13
Power2:58
Kilowatt-hour3:05
About me
My name is Thomas Schuurmans, and I have a passion for physics and for understanding why things work the way they do. What I want is to get young people asking “why is that?” and “how does that work?” more often, and to hand them the tools of physics in a way that is simple and that you can see.
I graduated in applied physics at Delft University of Technology in 2003. After that I spent more than twenty years outside education: first at TNO, the Dutch applied research institute, then at a design agency, and eventually founding Proportion Global, through which I work on innovation questions in Africa, Latin America and South Asia. That work resembles physics more than you would expect: don't start from a solution, first understand what is going on, try something, be wrong, and look again.
Since 2026 I have been teaching physics to both lower and upper secondary classes, and I started a master's at the University of Amsterdam for my full teaching qualification. That is where I learned that a secondary school pupil's real attention for new material lasts about seven minutes. And my own weakness happens to be telling too many side stories.
That is where the idea came from: videos that explain one topic sharply and visually, inside those seven minutes. I make them for my own pupils. Then I publish them, because good explanation should be within reach of anyone who needs it, wherever you live and whatever language you think and speak in.