Efficiency is the share of the energy that is used usefully: the useful energy divided by the input energy. The rest does not disappear, but almost always becomes heat. You may also divide the powers. An electric scooter that gets 350 watts and delivers 280 watts of motion has an efficiency of 0.80, or 80 percent.
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Of every five litres of petrol in a car, only about one goes into driving. Where does the rest go? That energy does not disappear: it becomes heat. The share of the energy that is used usefully is called the efficiency.
Energy is never lost. Whatever a device or an engine does not use for what you want, it converts into a form you did not want, almost always heat: in the exhaust, in the cooling system, in the engine itself. You draw that in a Sankey diagram. On the left the energy comes in: the input energy. One arrow goes to what you wanted, the useful energy, and the other bends away as heat. Together they are exactly as wide as what came in.
The efficiency, the Greek letter η (eta), is the useful energy divided by the input energy. That gives a number between 0 and 1, and times 100 turns it into a percentage. Because it works just as well per second, you may also divide the powers: the useful power divided by the input power. Always divide the useful part by what goes in, otherwise you end up above 100 percent.
In the worked example, an electric scooter gets 350 watts from its battery, and the motor turns 280 watts of that into motion: an efficiency of 0.80, or 80 percent. The car from the start comes to about 20 percent. Then you calculate one yourself: a drill that takes in 600 watts and delivers 450 watts to the drill bit. Halfway through there is a question and a silence: pause the video there and think it through first.
A higher efficiency is also part of the energy transition, the switch to energy that does not come from fossil fuels: alongside generating sustainably and using less energy, it is about devices that turn less energy into heat.
In this video
The full explanation, in writing
One in five
0:00Of every five litres of petrol in a car's tank, only about one goes into driving. The rest does not.
Energy goes nowhere
0:12Those four litres have not vanished: they become heat. Energy is never lost. What a device or an engine does not use for what you want, it converts into a form you did not want, almost always heat: in the exhaust, in the cooling, in the engine itself.
0:29You draw that in a Sankey diagram. On the left the energy comes in: the input energy. One arrow goes to what you wanted, the useful energy. The other arrow bends away: that becomes heat. Together they are exactly as wide as what came in.
Which share is useful
0:49The share of the energy used usefully is called the efficiency. Efficiency, the Greek letter eta, is the useful energy divided by the input energy. That gives a number between zero and one, and times a hundred makes it a percentage. Since it works just as well per second, you may also divide the powers: the useful power divided by the power that goes in.
1:16An electric scooter gets three hundred and fifty watts from its battery, and the motor turns two hundred and eighty watts into motion. Two hundred and eighty divided by three hundred and fifty is zero point eight zero: an efficiency of eighty percent.
1:33The car from the start gets about twenty. Your turn. A drill takes in six hundred watts, and delivers four hundred and fifty watts to the bit. What is the efficiency, and how many watts become heat?
Seventy-five percent
1:50Always divide the useful part by what goes in, not vice versa, or you end up above a hundred percent. Four hundred and fifty divided by six hundred is zero point seven five: seventy-five percent. The rest, a hundred and fifty watts, becomes heat, and you feel it when the drill gets warm after a while.
2:09And so a higher efficiency is part of the energy transition, the switch to energy that does not come from fossil fuels: alongside generating sustainably, it is about using less energy, and about devices with a higher efficiency.
Three things to remember
2:27Three things. Energy does not vanish: what is not useful almost always becomes heat. Efficiency is the useful energy divided by the energy that goes in, and that also works with powers. And a higher efficiency is part of the energy transition, alongside using less and generating sustainably.
Two questions
2:49Two questions. One. A laptop charger takes seventy watts from the socket and delivers sixty-five watts to the laptop. What is the efficiency? Two. An electric motor with ninety percent efficiency delivers four hundred and fifty watts of motion.
3:05How much electrical power does it take in?
The answers
3:11The first: sixty-five divided by seventy is zero point nine three, ninety-three percent. The second: the input power is the useful power divided by the efficiency. Four hundred and fifty divided by zero point nine zero is five hundred watts.
What a watt is
3:29Want to know what a watt actually is, and why "watts per hour" does not exist? Then watch the video about power on my YouTube channel.
Frequently asked questions
What is efficiency?
Efficiency is the share of the energy that a device uses usefully. You calculate it as the useful energy divided by the input energy. That gives a number between 0 and 1; times 100 gives a percentage. The symbol is the Greek letter η (eta).
How do you calculate efficiency with power?
Divide the useful power by the input power. An electric scooter that gets 350 watts from its battery and delivers 280 watts of motion has an efficiency of 280 divided by 350 is 0.80, or 80 percent.
Where does the energy go that is not used usefully?
It is not lost. A device or engine converts it into a form you did not want, almost always heat. In a car that heat is in the exhaust, the cooling system and the engine itself, and that is why a drill gets warm after a while.
Can efficiency be more than 100 percent?
No. No device ever puts out more useful energy than the energy that goes in. If you still end up above 100 percent, you have turned the division around: always divide the useful part by what goes in, and not the other way round.
What does efficiency have to do with the energy transition?
Alongside generating sustainably, the energy transition is also about using less energy. Devices with a higher efficiency turn less energy into heat, so they need less energy for the same job.
Next: what a watt actually is, and why "watts per hour" does not exist. That is in the video about power on this channel.
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Physics with Thomas: explained with pictures where they add something, and without them where they distract.
In this series
Electric currentthe electrons crawl, your lamp is on at once6:12
Voltage and capacitytwo batteries, the same number4:43
Connecting ammeter and voltmeterswapping them wrecks your meter4:10
Direct and alternating voltage and the diodewhy does an LED only work one way?4:11
Resistance and Ohm's lawdoes resistance double with voltage?4:21
Resistance of a wirewhy a long cable charges slowly4:40
Variable resistor, NTC, PTC and LDRresistors that change by themselves3:40
Series circuitwhich lamp gets the most current?3:32
Parallel circuitwhy does the resistance actually get smaller?3:56
Series-parallel circuitwhy adding up doesn't work here3:37
Circuits practical testhow to find the resistor under the tape5:13
Powerwhy is there no such thing as watts per hour?2:58
Kilowatt-hourwhat does your wifi router cost per year?3:05
Efficiencywhere does the rest of the energy go?3:43
Earthing and residual current devicewhat protects you from a shock?2:58
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.
Proportion Global now works with a network of local consultants in 67 countries, so I hardly need to travel myself any more. That gives me more time, and more of a need, to make a difference closer to home as well. So I picked up my old love for physics again. Since September 2026 I have been teaching physics two mornings a week at the Cartesius Lyceum in Amsterdam, to lower and upper secondary classes.
I am very much a visual learner myself when I want to learn something new. That is where the idea came from: videos that explain one topic sharply and visually, in just a few minutes. I make them for my own pupils. Then I publish them, because a good explanation should be within reach of anyone who needs it, wherever you live and whatever language you think and speak in.