What is Electrical Power: watts

Power is one of the key concepts and units associated with electrical science, measured in watts, power is an important parameter.

Electrical Power Includes:
What is power     Understanding Power Factor    

An important aspect of any electrical or electronic circuit is the power associated with it. It is found that when a current flows through a resistor, electrical energy is converted into heat.

This fact is used by electrical heaters which consist of a resistor through which current flows. Light bulbs use the same principle, heating the element up so that it glows white hot and produces light.

At other times much smaller resistors and very much smaller currents are used. Here the amount of heat generated may be very small. However if some current flows then some heat is generated. In this instance the heat generated represents the amount of electrical power being dissipated.

Definition of power

Whether power is used in a mechanical environment or an electrical environment, the definition of power is still the same. The way in which it may be discussed may be slightly different, but nevertheless the definition and actuality of it is exactly the same.

Electric power definition:

Electric power is the rate, per unit time, at which electrical energy is transferred by an electric circuit. It is the rate of doing work.

In terms of an electric circuit, electrical power is the rate, per unit time, at which electrical energy is transferred by an electric circuit.

From the definition it can be seen that:

W   = V   Q t

But as:

Q t = Current, I

Substituting:

W = V   I

Where:
    W = power in watts
    V = potential in volts
    I = current in amps
    Q = charge in coulombs
    t = time in seconds

What is a watt: unit of power

The unit of power is the watt which is denoted by the symbol W and it is named after the Scottish engineer James Watt (1736–1819).

Definition of the watt:

The watt is the SI unit of power defining the rate of energy conversion and it is equivalent to one joule per second.

The watt can be defined according to the application:

  • Electrical definition of the watt:   one watt is the rate at which work is done when a current of one ampere, I of current flows through a network which has an electrical potential difference of one volt, V. W = V I
  • Mechanical definition of the watt:   one watt is the rate at which work is done when the velocity of an object is held constant at one metre per second against constant opposing force of one newton.

Like many other SI units, there are multiples and sub-multiples as the range of power levels can vary from minute levels of radiation received on radio antennas from distant stars, through to the enormous levels generated by large electricity power stations.

Multiples & Submultiples of watts
Current Name Abbreviation
10-15 watts femtowatts fW
10-12 watts picowatts pW
10-9 watts nanowatts nW
10-6 watts microwatts µW
10-3 watts milliwatts mW
watts watts W
103 watts kilowatts kW
106 watts Megawatts MW

It often helps to have a view of the typical power levels of various items that are mentioned in association with electronic and electrical systems.

Some examples of the typical power levels are given in the table below.

Typical Power Levels of Various Electrical and Electronic Devices & Systems
Device Details
Electric fire Typically 1 kW per bar
Desktop computer normally less than 100W
Kettle Typical 2.5 kW
42 inch flat screen LED Television ~100W
Domestic incandescent light bulb Up to 150 W
Domstic LED light bulb Up to 20 W

Calculating power

The amount of power dissipated in a circuit can be easily determined. It is simply the product of the potential difference or voltage across the particular element, multiplied by the current flowing through it. In other words an electrical fire running from a 250 volt supply, and consuming 4 amps of current will dissipate 250 x 4 = 1000 watts or 1 kilowatt. In other words.

W = V   I

In some instances the actual resistance of the circuit element may be known. By using Ohm's Law ( V = I x R) it is possible to calculate the power if either the voltage or current is known. For example the mains voltage may be known to be 250 volts and the element resistance may be known to be 62.5 Ohms.

By performing some simple algebra it is possible to discover the very useful formulae:

W = V 2 R

. . and . .

W = I 2   R

Using these formulae it is simple to work out the power dissipated in the 62.5 ohm resistor when a voltage of 250 volts is placed across it

W = V 2 R = 250 2 62.5 = 1000 W


Power is one of the key units in many electronic circuits. It can be used to indicate the level of heat dissipated in a unit or even an individual component, it can be used to define the power consumed, and it can also be used to define the amount of power generated by the system to pass on to the next item. In these and very many other areas, power measured in watts is a key parameter which is of great importance.

Ian Poole   Written by Ian Poole .
  Experienced electronics engineer and author.



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