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Efficiency = useful output energy ÷ total input energy, or efficiency = useful power output ÷ total power input. Free GCSE Physics notes and 14 FAQs.
Calculating Efficiency is part of Conservation and dissipation of energy in GCSE AQA Higher Physics.
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Efficiency = useful output energy ÷ total input energy, or efficiency = useful power output ÷ total power input. Free GCSE Physics notes and 14 FAQs.
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Use this lesson to stay inside Conservation and dissipation of energy while connecting the detail back to the wider Energy topic.
Read the core explanations from Calculating Efficiency before testing yourself from memory.
Your phone battery dies. Your car needs petrol. Your electricity bill arrives. All these problems come down to one thing: energy isn't perfectly transferred from one form to another.
When you charge your phone, not all the electrical energy becomes stored chemical energy in the battery. Some becomes heat in the charger. When your car burns petrol, not all the chemical energy becomes kinetic energy. Most becomes waste heat through the exhaust.
This is where efficiency comes in. It tells us exactly how much of our input energy actually does what we want it to do.
Think of efficiency like this: if you put 100 J of energy into a system, how much useful energy comes out? A perfect system would give you all 100 J back in the form you want. Real systems give you less.
The missing energy hasn't disappeared - that would violate energy conservation. It's been transferred to unwanted forms, usually heat and sound. Your car engine is about 25% efficient. That means 75% of the petrol's energy becomes waste heat, not motion.
There are two ways to calculate efficiency, depending on whether you're given energy values or power values. Both give the same answer because power is just energy per second.
Notice that efficiency has no units - it's just a number. You can express it as a decimal (0.25) or percentage (25%). Both equations give identical results because power = energy ÷ time, so the time cancels out.
Let's work through some typical exam calculations. The key is identifying what energy is useful versus what's wasted.
Worked example: Energy Transfer Efficiency
An electric motor uses 2000 J of electrical energy to lift a weight. 1600 J is transferred to gravitational potential energy. Calculate the efficiency. [3 marks]
Answer: 0.8 or 80%
Worked example: Power-Based Efficiency
A power station has an input power of 1000 MW and outputs 350 MW of electrical power. Calculate its efficiency. [3 marks]
Answer: 0.35 or 35%
Different energy transfer processes have very different efficiencies. Understanding these helps explain why we use certain technologies and avoid others.
LED lights are about 80% efficient - most electrical energy becomes light. Old filament bulbs are only 5% efficient - most energy becomes unwanted heat. That's why LEDs are replacing filament bulbs everywhere.
Electric cars are about 90% efficient at converting stored electrical energy to kinetic energy. Petrol cars are only 25% efficient. This huge difference explains why electric cars need much less energy per mile.
Efficiency questions appear frequently in GCSE exams. Here's how to maximise your marks and avoid the traps that catch most students.
The biggest mistake students make is confusing useful output with wasted energy. If a question says '400 J is wasted as heat' and the input is 2000 J, the useful output is 2000 - 400 = 1600 J, not 400 J.
Another common error is using the wrong equation. If you're given 'The motor has an input power of 500 W', use the power equation. If you're given 'The motor uses 500 J of energy', use the energy equation.
Efficiency calculations are most useful when you compare what you want from a system with what is wasted. If a device is 80% efficient, that means 80% of the input becomes useful output and 20% is transferred in unwanted ways, often as heating. This helps explain why improving insulation, reducing friction and using better materials can reduce waste without breaking energy conservation.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Efficiency = useful output energy ÷ total input energy, or efficiency = useful power output ÷ total power input. Both give the same answer because power is just energy per second, and efficiency has no units: express it as a decimal (0.25) or a percentage (25%).
To calculate efficiency, divide the useful output by the total input. An electric motor using 2000 J of electrical energy to transfer 1600 J to gravitational potential energy has an efficiency of 1600 ÷ 2000 = 0.8, or 80%.
Efficiency is the fraction of input energy that is transferred to useful output energy. It is always less than 1 (or 100%) because some energy is always wasted, usually as heat and sound. A car engine is about 25% efficient, so 75% of the petrol's energy becomes waste heat.
Energy efficiency = useful output energy ÷ total input energy, or useful power output ÷ total power input when power values are given. Both give the same answer because power is energy per second, so the time cancels out.
Efficiency is always less than 1, or 100%, because some energy is always wasted. The missing energy has not disappeared, which would violate energy conservation; it has been transferred to unwanted forms, usually heat and sound.
To calculate efficiency using power, divide the useful power output by the total power input. A power station with an input power of 1000 MW and an output of 350 MW of electrical power has an efficiency of 350 ÷ 1000 = 0.35, or 35%, with 650 MW wasted as heat.
LED lights are about 80% efficient, so most electrical energy becomes light, while old filament bulbs are only 5% efficient, with most energy becoming unwanted heat. That is why LEDs are replacing filament bulbs everywhere.
Efficiency has no units; it is just a number. It can be expressed as a decimal, such as 0.25, or as a percentage, such as 25%, and it should be given in the format the question asks for.
A petrol car engine is about 25% efficient, meaning 75% of the petrol's energy becomes waste heat, not motion. Electric cars are about 90% efficient at converting stored electrical energy to kinetic energy, so they need much less energy per mile.
The efficiency of a power station is useful power output ÷ total power input. A power station with an input of 1000 MW and an electrical output of 350 MW has an efficiency of 350 ÷ 1000 = 0.35, or 35%, with 650 MW wasted as heat.
The useful output energy is the total input energy minus the wasted energy. If the input is 2000 J and 400 J is wasted as heat, the useful output is 2000 − 400 = 1600 J, not 400 J; confusing useful and wasted energy is the biggest mistake in efficiency questions.
A device that is 80% efficient transfers 80% of its input energy into useful output, while the other 20% is transferred in unwanted ways, often as heating. LED lights are about 80% efficient.
The energy version of the efficiency equation is used when values are given in joules, and the power version when they are given in watts. A motor with an input power of 500 W needs the power equation; a motor that uses 500 J of energy needs the energy equation.
Old filament bulbs are only about 5% efficient, because most of their electrical energy becomes unwanted heat. LED lights are about 80% efficient, which is why LEDs are replacing filament bulbs everywhere.