Electrical Power Calculator

Created by Miłosz Panfil, PhD
Last updated: Sep 09, 2021

This Electrical Power Calculator helps you compute the power consumed by electrical devices. Do you want to know how to calculate electrical power? Are you wondering: what is the power factor? Just read the text below to find out.

How to calculate electrical power

The electrical power, just like mechanical power, is the amount of work performed in a unit of time. In electrical circuits, work is performed by an electric current. The power depends on the "amount of workers available in a unit of time" - the current I and the energy of "a single worker" - the voltage V. In a DC circuit, the power is

P = I * V,


  • I [A] is the current,
  • V [V] is the voltage,
  • P [W] is the power.

In the AC circuits, the equation is

P = I * V * PF,

where the new symbol PF stands for power factor.

What is the power factor

In AC currents both the current and voltage vary periodically in time. The values I or V correspond to the root mean square (RMS). RMS is a square root of the mean of squares of numbers. The commonly referred voltage of electrical outlets (230 V in EU and Australia, 110 V in the USA and Canada, 100 V in Japan) is the RMS voltage. In the AC circuits, the current and voltage might not be in phase. The maximum value of the current might be ahead of or lagging behind the maximum value of the voltage. This makes the transfer of power less effective. In the worst case, when the current and voltage are entirely out of sync, the transmitted power is zero.

The power factor tells us how synchronized is current with the voltage. If they're in sync, the power factor is 1. Otherwise, it is less than one, reaching zero in the completely out-of-sync case. The power factor depends on the device. For a device that is purely resistive, like an electric kettle or an electric heater, the power factor is 1. A device with inductive or capacitive elements puts the current and voltage out of the phase. This makes it power factor less than 1. Check the power factor calculator to learn more.

Electrical power calculator

To compute the electrical power you have to specify the current, voltage, and the power factor. For devices connected to the electrical outlets, the voltage is just the voltage of the domestic power. The current drawn by the device can be usually found either marked on a plug or somewhere on the device. The power factor is a bit trickier to find - unless you have a Power Quality Analyzer at hand. Check this list for power factors of a few typical household devices:

  • lamps with a standard bulb: PF = 1,
  • fluorescent lamps: PF = 0.93,
  • common induction motor at half load: PF = 0.73, at full load: PF = 0.85,
  • electric oven (with resistive heating element): PF = 1.0,
  • inductive oven: PF = 0.85.

The exact value of the power factor depends on the details of the construction, so take these values with a grain of salt.

Miłosz Panfil, PhD
Power factor
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