Electric current

Watt's Law: Formula, Relationship with Ohm's Law and Use

Watt's law: formula, relationship with Ohm's law and use

Watt's law defines the relationship between power, amperage, and voltage in an electrical circuit. Watt's law states that: "the power of an electrical circuit or device is the product of its voltage and the current flowing through it."

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This electrical energy consumption of a circuit is determined by the power. In turn, the power depends on the resistance of the apparatus, the input voltage and the current that flows through the resistance.

Watt's Law Formula

Watt's law formula is presented as follows:

P = V I


  • P is the electrical power of an electrical resistor.

  • V is the voltage applied to the resistance of a circuit.

  • I is the amperage.

The consumption of electrical power is measured in watts (W), voltage in volts (V) and the unit of measurement for amperage is the ampere (A).

In this formula it can be verified that the power consumed is directly proportional to the difference in input and output voltage and the current intensity.

There are three possible formulas for calculating electrical power in a circuit.

Watt's law: formula, relationship with Ohm's law and use

What Is the Relationship Between Watt's Law and Ohm's Law?

Watt's law relates power to voltage and current intensity. On the other hand, Ohm's law relates the voltage to the current intensity and the electrical resistance of a circuit.

Ohm's law formula is:

V = I · R


  • V is the voltage.

  • I is the current intensity.

  • R is the electrical resistance offered by a circuit measured in ohms.

Since both laws use the same electrical quantities, they are often combined to get some useful equations.

What Is Watt's Law For?

Some of the applications of Watt's law include:

  • Measurement of the actual amount of energy that an electric generator can produce.

  • Measurement of the electrical power that a building can consume.

  • If the power and voltage of the electrical component are known, the amperage can be measured. On the other hand, the voltage can be obtained if the power and current intensity are known.

  • The formulas obtained from the combination of Watt's law and Ohm's law can be applied to determine the electrical resistance of a component.


Published: December 16, 2021
Last review: December 15, 2021