Key Concepts: Power & Efficiency

Power

• Power is the rate of doing work or the rate of energy transfer: P = W/t = ΔE/t.
• SI unit: Watt (W). 1 W = 1 J/s.
• Alternative formula: P = Fv (force × velocity) for constant force along the direction of motion.

Efficiency

• Efficiency measures how much input energy is converted to useful output:
• η = (useful energy output / total energy input) × 100%.
• Efficiency is always ≤ 100%. No real machine is 100% efficient due to friction, heat loss, etc.

Examples

• A 60 W light bulb converts 60 J of electrical energy every second (mostly to heat in incandescent bulbs).
• A car engine might be 25% efficient — only 25% of fuel energy becomes motion; the rest is lost as heat.
• LED bulbs are ~90% efficient at converting electrical energy to light.