Electrical power

Martin McBride, 2020-07-08
Tags power
Categories electronics electricity

We are used to talking about electrical power - the power of a lightbulb, an electric heater, or in audio terms the power of a set of speakers.

Power is measured in watts. For a DC circuit (eg something powered by batteries), the power is simply the voltage multiplied by the current:

P = V * I

We will look at this in a bit more detail below.


Energy takes many forms (kinetic, thermal, chemical, and so on). Energy represents work done, or the capacity to do work.

For example, suppose you were to pick up an apple from the ground, and lift up by 1m. This takes work, because you are pulling the apple up against the force of gravity. Energy is measured in joules, and it takes about 1 joule to lift a typical apple by 1m.

The apple now has 1 joule of potential energy. If you let the apple fall, that potential energy (the work you did in lifting it) is converted into kinetic energy (speed).

Electrical energy

A battery uses a chemical reaction to separate positive and negative charges (it separates off negative electrons and leaves positive charged atoms behind). This takes work, because the positive and negative charges are attracted to each other. The energy to do this comes from the chemical energy of the materials inside the battery.

The battery separating the charges is similar you you lifting the apple off the ground.

Rather than lifting a weight to a particular height, the battery raises electrons to a certain voltage.

The charge of one electron is tiny, so we measure charge in coulombs. One coulomb is equivalent to the charge of 6.241e18 electrons!

Electrical energy is equal to the amount of charge in coulombs multiplied by the voltage it is raised to. 1 coulomb of charge raised to 1 volt has 1 jouile of energy.


Power is simply the rate that energy is consumed, in other words the amount of energy consumed per second. If a light bulb, for example, consumes 1 joule of energy per second, we say it is a 1 watt bulb.

Electrical power

Since electrical energy is defined as voltage multiplied by charge, how is electrical power defined?

Well it is defined as voltage multiplied by the amount of charge per second that flows.

The amount of charge per second is better known as the current. A current of 1 amp means that 1 coulomb of charge is flowing per second. So the power is given by:

P = V * I

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