The efficiency formula
Efficiency is useful output divided by total input, expressed as a percent: η = output ÷ input × 100. Getting 750 J of useful work from 1000 J of input is 75% efficient. The rest is usually lost as heat. To compare the power involved, see the horsepower calculator.
The word useful is doing the work in that definition. Energy is never destroyed, so the missing 250 J still exists — it has simply gone into friction, sound and waste heat rather than the job you wanted done.
How to use this calculator
Enter the output energy and the input energy in the same units (such as joules). The result is efficiency as a percentage. Input must be greater than zero. Real machines are always below 100% because some energy is lost. Compare the work done with the work calculator.
A result above 100% is a signal that something is wrong with the inputs, not a discovery: usually the two figures are in different units, or an energy total has been swapped with a power reading.
Worked example
A machine takes in 1000 J and delivers 750 J of useful output: 750 ÷ 1000 × 100 = 75%. Raise the useful output to 900 J and efficiency climbs to 90%, since efficiency rises with useful output for fixed input.
Interpreting the figure depends on the device: an electric motor at 90% is ordinary, while a petrol engine near 30% is typical. Chaining devices multiplies their efficiencies, so a 75% motor driving a 75% gearbox delivers about 56% overall. Efficiency compares outputs over the same period, so express both as power if the timings differ.
Frequently asked questions
- How do I calculate efficiency?
- Divide useful output by total input and multiply by 100: η = out ÷ in × 100.
- Why must the input not be zero?
- Dividing by zero is undefined, so efficiency has no value when input is zero.
- Can efficiency exceed 100%?
- No. Real systems always lose some energy, so efficiency is always below 100%.
- What units should I use?
- Use the same energy or power units for output and input; the ratio is unit-free.
- Can I use power in watts instead of energy in joules?
- Yes, as long as both figures are power. Efficiency is a ratio, so watts divided by watts works exactly like joules divided by joules. What you must not do is mix a power output with an energy input, which would produce a meaningless number.