By James Marshall, updated March 24, 2022
Power is the rate at which work is performed. A watt measures electromechanical power and is defined as one ampere of current flowing through a circuit with a voltage difference of one volt. An ampere represents one coulomb of charge passing through a point in the circuit each second. To calculate a circuit’s wattage, you need to know both its amperage and voltage.
Determine the voltage of the circuit. For this example, we’ll use 110 V, the standard residential voltage in the United States.
Determine the amperage of the circuit. In this illustration, a light bulb draws about 0.91 A.
Multiply the amperage by the voltage to obtain the wattage. The relationship is expressed by the equation W = A × V, where W is watts, A is amperage, and V is voltage. Using the example values: 110 V × 0.91 A = 100 W. Thus, the light bulb consumes 100 W.
For accurate results, always verify your voltage and amperage readings with calibrated instruments and ensure the circuit is properly isolated before measurement.
