Watt's law calculator

Our Watt's law calculator automatically calculates Current, Resistance and Effect. Also learn what Watt's law is all about.
Electrical cables in factory building

Electrical cables

Watt's law

What is Watt's law actually about? Read the article and understand the basics.
Watt's law is a formula used to calculate electrical effect in an electric system.

The formula is P=VxI

P is Watt

V is Volt

I is Ampere

To calculate the Watt (P) one has to multiply Volt (V) with Ampere (A)

To calculate the Volt one has to divide Watt (P) with Ampere (I)

To calculate the Ampere (I) one has to divide Watt (P) with Volt (V)

A good way to show how the formula works is to calculate the effect of an electric motor. To keep it simple, lets consider a 2-phase motor at first and then consider a 3-phase motor. Please note we also consider the motors efficiency which is described below and include this in the calculation. 

Calculate effect of electric motor

2-phase motor

Let’s say that you have 230 Volts in your home, and your fuse is 16 Amperes. We will now divide 230 with 16, and the result we get is 3680 Watt, and that’s how much effect your motor performs with these parameters.

But let’s say that you have a 400 Volt system with 16 Amperes in your house instead, and the same motor can be used at this currency as well? We will now divide 400 with 16 and the result we get is 6400 Watt.

As we can see the formula shows that Watt (effect) is dependent on Volt and Ampere.

3-phase motor

The formula starts in the same manner for a 3-phase motor, but we need to divide the square root of 3 as well due to the internal configuration of the 3-phase motor.

The formula now looks like this P=(VxI)x√3


Additionally, we recommend that you calculate the efficiency of the motor as well. The reason is that no motor operates at 100% efficiency. The motor will for example become hot, and some of the energy will simply be lost as heat. The efficiency is normally stated on the motors panel, and the term is «Cosinus Phi». Cosinus Phi can, for example, be 0,87, meaning that the motor only delivers 87% of the nominal effect. The sign for Cosinus Phi looks like this: φ

The formula now looks like this P=((UxI)x√3)xφ

The Watt's law calculator

If you feel that the formula is hard to grasp we have made it easier for you. Simply use our calculator step by step, fill in the required information and calculate to see the answer.

Did you know?

In physics, Effect is defined as amount of work per unit of time. The unit for effect in the SI-system is Watt and the unit for work is Joule. One watt is equal to one joule per second. Since 1 joule is equivalent to the amount of work it takes to move 1 newton over a distance of 1 meter, 1 watt is equal to move an object of 1 newton over a distance of 1 meter per second. To say it in an understandable manner, this is approximately the work required to lift 102 grams 1 meter upwards over a period of 1 second as the surface of the earth.
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