Efficiency
Find the percentage of input energy a machine turns into useful output.
Use the same units for both (joules for energy, watts for power) — the units cancel and efficiency comes out as a pure percentage.
Real machines are always below 100%; the rest is lost as heat, sound, or friction.
Efficiency is the share of energy that does useful work: η = (useful output ÷ total input) × 100%. A machine that delivers 60 J of useful output from 100 J of input is 60% efficient. Output and input must be in the same units (joules or watts), so the result is a pure percentage.
What efficiency means
Efficiency tells you how much of the energy (or power) you put into a device comes back out as the result you actually wanted. It is a ratio of useful output to total input, written as a percentage. Because both quantities are energies or powers measured in the same units, those units cancel and efficiency is a plain number between 0% and 100%.
η = efficiency (%); output and input in the same units — joules (J) for energy or watts (W) for power
Worked example
A small motor draws 100 J of electrical energy and delivers 60 J of mechanical work; the rest warms the windings. How efficient is it?
- 1 Identify useful output and total input. Useful output = 60 J of mechanical work; total input = 100 J of electrical energy.
- 2 Use the same units for both. Both are in joules, so the units cancel. If one were in watts, convert so both match.
- 3 Divide output by input. 60 ÷ 100 = 0.6 — the fraction of energy that became useful.
- 4 Multiply by 100 for a percentage. 0.6 × 100 = 60%, so the motor is 60% efficient and wastes 40 J as heat.
Typical efficiencies
Approximate real-world values — all below 100%.
| Device | Typical efficiency |
|---|---|
| LED bulb | ≈ 80–90% |
| Electric motor | ≈ 90% |
| Gas car engine | ≈ 25–30% |
| Incandescent bulb | ≈ 5% |
Why efficiency is always below 100%
No real machine reaches 100%. Some of the input energy always escapes as heat, sound, light, or friction instead of doing the useful job — a car engine warms up, a motor hums, a bulb glows hot. That lost energy is not destroyed; energy is conserved, it just leaves in a form you cannot use, which is why output is always smaller than input.
Watch your units. Output and input must be the same kind of quantity in the same units — joules with joules, or watts with watts. Mixing energy with power, or joules with kilowatt-hours, gives a meaningless number. If your result comes out above 100%, the output or input figure is wrong, because that would require creating energy from nothing.