What is the basic unit of electrical power?

• A. Ohm
• B. Watt
• C. Volt
• D. Ampere

Answer: (B)

Power is the rate at which energy is transferred in a circuit, and the standard SI unit for this rate is the watt. This unit comes from the relationship P = V × I, where voltage (potential difference) is measured in volts and current is measured in amperes. Since a volt times an ampere equals a joule per second, the watt cleanly expresses power. That’s why the watt is the correct unit for electrical power.

The other units describe different electrical quantities: the ohm measures resistance, the volt measures potential difference, and the ampere measures current. They aren’t units of power, so they don’t fit as the basic unit of electrical power. For example, if you have 120 volts flowing with 2 amperes, the power is 240 watts.