What is Watts Law : Working, Problems & Its Applications

There are many theories and laws that exist in the study of electronics that allow us to know the workings of electronic components & electric circuits. So, watts law is one such law which is stated by a chemist & a Scottish engineer namely “James Watt”. The name of the watts law is taken after James Watt.

Watt’s Law states the most basic relationships within electronics like the main relationship between the quantities & power defined through Ohm’s law. If we understood these concepts, the study of electronics will become very simple. This article discusses an overview of Watts Law and its statement with examples.

What is Watts Law?

Watts Law Definition: Watts law states that the relationship between different parameters such as; voltage (V), current (I) & power (P) in a circuit. The power within a circuit is the product of voltage & current. Watts’s law formula provides the main relationship between P (watts), V (Volt) & I (amps).

P = V * I (Power = Voltage X Current)

V = P/I (Voltage = Power/Current)

I = P/V (Current = Power/Voltage)


In order to understand the concept of Watts Law, understanding the following parameters is necessary.

Voltage (V)

The voltage or ‘V’ in an electrical circuit is the potential difference between two points. So any difference within electrical potential mainly causes electrons to flow from a high potential point to a low potential point. Here, the unity of the voltage is ‘V’.

Current (I)

The current or ‘I’ is the amount of flow of electric charge throughout a point within a circuit in a fixed time. The unit of current is ampere (A), or “amps”. When there is a potential difference within the circuit then only current flows.

Resistance (R)

Resistance is the restriction of the flow of current and it can be measured in ohms or ‘Ω’. In an electrical circuit, it is the measurement of a component’s ability to restrict the current flow. This law specifies the main relationship between voltage, current & resistance. The flow of current through a conductor is proportional directly to its voltage (I=V/R).

Power (P)

Power (P) is a component that can use for each unit time. Simply, it is the sum of electrical energy transmitted for each unit of time. So, the unit of power (P) is joule/second or J/sec, measured in watts (W).

The main difference between Watt’s law and Ohm’s law (Watts Law Vs. Ohms Law) is; Ohm’s law is the main relationship between R (resistance), V (voltage) & I (current) within a circuit. Watt’s law is the main relationship between P (power), V (voltage) & I (current).

These two laws can be combined to obtain useful formulas.

Ohms Law = V = IR => I = V/R => V = IR.

Substitute the above values in Watts Law, we can get

P = I* IR => I2R

P = V*V/R => V2/R

From the above formulas, we can derive many formulas based on our requirements.

Watts Law Triangle

Both the ohms law and watts law include similar electrical quantities; so they can be easily combined to derive some helpful equations. The basic equation of Ohms Law is reversed for power and it provides the same equation with some combinations to discover different individual quantities.

Watts Law Statement
Watts Law Statement

Here, electrical power in a circuit can be calculated through three possible formulas from the above watts law triangle. If the measured power is positive, then the equipment uses power. Similarly, if the measured power is negative then the component generates power.

The values that can be calculated using Power Triangle are;

P = I X V

I = P/V

V = P/I

Ohms Law Wheel

Ohm’s law wheel includes another variable like power (P) which is calculated in Watts. So power can be defined as the rate energy which is transferred through the circuit for each unit of time that is expressed within watts. The ohms law wheel merges the PIE chart using h Ohm’s law.

Ohms Law Circle
Ohms Law Circle
  • The wheel formula may seem complex but it is very easy to use like the PIE chart.
  • In the chart, every quadrant includes different formulas that signify the main relationship between two variables like they are proportional directly or not. In every quadrant, we can find three formulas.
  • Identify, what you are trying to solve for resistance, voltage, current, or power.
  • Need to check the known values because you need a minimum of two values
  • Discover the formula section in the wheel to find out the values.
  • While doing calculations, compatible values need to use. For instance, kilohms must be changed into ohms; mA (milliamperes) must be changed into amperes (A).

Watts Law Solved Problems

1). A home light bulb is 60 W then the applied voltage toward the light bulb is generally 110V/220V so, measure the consumed current.

We know that I = P/V = 60 W / 110V = 0.54 Amps

I = P/V = 60W / 220V = 0.27 Amps.

2). If the voltage of the light bulb is 110 volts & the power is 60 W, then what is the current?

Here, the bulb’s voltage and power are 100V and 50W correspondingly.

We know that I = P/V

So, by substituting the above-mentioned values

I = 50/100 = 0.5 Amperes

3). Calculate the power using the watts law once 110 V generates 3 A of current.

We know that P = VI => 110V * 3A = 330W

4). Measure the amount of power with watts law once the current flow throughout a 2kΩ of the resistor is 4 A.

We know that P = I2R => (4A)2(2000 Ω)

= 16A2 (2000 Ω) = 32,000 W

What is Watts Law used for?

The applications of Watt’s law include the following.

  • This law is used to measure the actual power if you have a source of power
  • It can be used to measure the requirement of power for just a single component. Once the voltage & current of the source are provided, then the values can be simply multiplied.
  • The power necessary for a building can be calculated with this law but it is very essential to guess the whole power requirement while designing the wiring in a building.
  • If you identify the voltage & the power for the component, then we can calculate the flow of current through the formula of Watt like I = P/V. Similarly, if we know the power and current then the voltage can be measured.
  • The combinations of ohms and watts law formulas can be used to decide the resistance for a component within the circuit.


The advantages of Watt’s law include the following.

  • This law is very useful in discovering the main relationship between different parameters like voltage, current, and power.
  • Watt’s law can be used to discover or analyze the resistance, ampere, voltage & power.
  • Watts can be used to decide battery performance.
  • Wattage can be used for rating the 5 watts or 10 watts light bulbs.

Is watts is a power or energy?

The power of the unit is the watt (W) otherwise radiant flux.

What is the formula for Ohm’s Law & Joule’s Law?

The joul formula is H = I x V x T and Ohms Law formula is V = I x R.

What is the current formula?

The current (I) formula is I = V/R, where the SI unit of I’ is Amp.

What is the difference between Ohm’s Law and Watts Law?

Ohm’s law describes the relationship between R, V & I within a circuit whereas Watt’s law describes the relationship between P, V & I.

What is meant by the power-law triangle?

The power-law triangle represents a right angle triangle to illustrate the main relation between reactive power, apparent power & active power.

Thus, this is all about an overview of Watt’s law and its statement with examples. This law states that the main relationship between different parameters likes voltage, resistance & current. So the power within a circuit is a product of the current & the voltage. There are many practical applications by using this law (P=IV). Here is a question for you, what are the limitations of watts law?