Which expression represents efficiency?

• A. Efficiency = (useful power output / total power input) × 100
• B. Efficiency = useful work / total energy × 100
• C. Efficiency = total power input / useful power output
• D. Efficiency = useful energy / total energy × 100

Answer: (A)

Efficiency is the fraction of input energy or power that actually becomes useful output energy or power per unit time. When we talk about how well a device converts energy, we compare the useful output to what goes in, per unit time. That makes the expression useful power output divided by total power input, all times 100 to turn it into a percentage. It’s the correct form because it uses the same kind of quantity (power) for both parts, so the units cancel and you get a meaningful percent.

For example, if a machine uses 60 watts of power and delivers 12 watts as useful output, the efficiency is (12 / 60) × 100 = 20%. The other forms mix different quantities (like energy or work) or invert the ratio, which doesn’t represent how efficiently energy is being converted in real-time.