Which units are used for energy and for power?

• A. Energy — Newton; Power — Volt.
• B. Energy — Joule; Power — Watt.
• C. Energy — Calorie; Power — Horsepower.
• D. Energy — Joule; Power — Ampere.

Answer: (B)

Energy is the capacity to do work, so it uses the joule (J) as its unit in the SI system. Power is the rate at which energy is transferred or work is done, and it uses the watt (W), where 1 watt equals 1 joule per second. Other units exist—calories are another energy unit (1 cal ≈ 4.184 J), and horsepower is a power unit often used for engines (1 hp ≈ 746 W)—but those are not the standard SI pair. Newton is a unit of force, volt is electric potential, and ampere is electric current, none of which are energy or power units.