Energy is always conserved — but not all of it ends up doing the JOB you want. Efficiency measures the fraction that does:
efficiency=total energy inputuseful energy output×100%
Some examples:
- LED lamp: ~90% efficient (most input energy becomes light).
- Incandescent bulb: ~5% efficient (most becomes heat).
- Petrol car: ~25% (most fuel energy lost as heat in the engine and exhaust).
- Electric motor: 80-95% (most input becomes mechanical work).
- Human body (working hard): ~25% (rest becomes body heat).
The wasted energy isn't gone — it just ends up in less useful stores. A Sankey diagram shows this beautifully: arrow width = amount of energy.
Worked example. A motor takes in 200 J of electrical energy and produces 160 J of kinetic energy. Find the efficiency.
Efficiency = 160 / 200 × 100 = 80%. The other 40 J becomes thermal energy in the wires, bearings and surroundings.