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Efficiency
Beginner
Useful output divided by input energy.
η = P_out/P_inEfficiency is useful output divided by total input: η = P_out/P_in ≤ 1. Losses are mostly heat. Real devices are always below 100%.
An engine using 300 J of fuel to do 75 J of work has η = 25%.
Efficiency doesn't tell you how much power is delivered.
In more detail
Efficiency is the ratio of useful energy or power output to the total energy or power input, usually given as a percentage. No real machine has 100% efficiency because of losses.
Real-world uses
- Car engines are typically around 25 to 35% efficient, with much of the energy lost as heat.
- LED bulbs are more efficient than incandescent bulbs at producing light.
- A petrol car may convert only about 20 to 30 percent of fuel energy into motion, so its efficiency is low.
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Frequently asked questions
Can an efficiency be above 100%?
No. Efficiency cannot exceed 100% because energy is conserved.
What is the most efficient engine?
Carnot engines set the theoretical upper limit, depending on the temperatures involved.
Can efficiency be more than 100 percent?
No. Useful output can never exceed input energy, so efficiency is always between 0 and 1, or 0 and 100 percent.
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