What is Watt’s law?

In the study of electronics, many laws and theories exist. These laws enable us to understand the workings of electric circuits and components. One such law is Watt’s law. Watt’s law is named after James Watt, a Scottish engineer and chemist. It defines the relationship between power, voltage and current. This law states that the power in a circuit is a product of voltage and current. In this article, we will discuss Watt’s law, its formula, applications, and other related concepts.

Important inputs in the definition of Watt’s law

In order for us to fully understand Watt’s law, we need to first discuss the parameters used in defining it which are:


Voltage (V) is the electrical potential difference between two points in an electric circuit. Any difference in electrical potential causes electrons to flow from a point of higher potential to a point of lower potential. Volt is the unit of voltage (V).


The current (I) is the amount of electric charge flowing through a point in a circuit at any given time. Its unit is ampere, or “amps” (A). Current can only flow when there is a difference in electrical potential.


Resistance (R) is the opposition to the flow of current. It is a measure of an electrical component’s ability to resist the flow of electrical current. The unit of measure is ohms (Ω).

Ohm’s law specifies the relationship between current, resistance, and voltage. This law states that the current passing through a conductor is directly proportional to its voltage, i.e., I=V/R


Power (P) is a measure of the quantity of work a circuit can do, or a component can consume per unit time. Simply put, it is the amount of electrical energy transferred per unit time. The unit of power is joule per second (J/sec) also known as watts (W).

Watt’s law formula

The formula for Watt’s law is: P=IV


I = current

P = power

V = voltage

People often ask; what is the difference between Watt’s law and Ohm’s law? While Ohm’s law defines the relationship between resistance, voltage, and current in a circuit; Watt’s law defines the relationship between power, voltage and current.

However, you can combine these laws to get useful formulas. According to Ohm’s law, I=V/R and V=IR. Substituting these into Watt’s formula, we get:

P=IxIR=I2R and P=VxV/R=V2/R

These formulas can also be used to derive several other formulas.

You can carry out most calculations involving Watt’s formula manually. However, you can also use an online Watt’s law calculator.

Applications of Watt’s law

The following are some of the applications of the Watt’s law formula:

Assuming the voltage supplied to that room is x volts, and the bulb is rated at y watts, then the resistance, R=x/y2.