Back to basics - the Watt (or kW) 

By on

When thinking about watts (W) or kilowatt (kW = 1000 W) it can be useful too keep in mind the fundamental ideas behind the unit. Watt is not a pure electrical or mechanical unit, but is a measure of the rate of doing work.

Let start at the beginning and understand the meaning of work.  This is effectively what the word says; doing work involves doing something - like moving things from one place to the other, climbing stairs, etc. In physics this is equated to the energy used in in transferring a force through a distance in the direction of the force. The SI unit of work is the Joule.

Another more general way to look at work it to consider that if the force acting on an object changes it's kinetic energy then the work done is equal to the change in kinetic energy. 

So does work = force x distance? Sometimes. Work is a scalar quantity and this equation holds if the force is acting in the same direction as the movement. If the force and distance are not acting in the same direction then we need to take the dot product of the force and distance vectors:


James Watt
19 January 1736 - 19 August 1819

myElectrical Equation 

If the angle θ is zero the work is simply the force multiplied by distance If the force is perpendicular to the distance, cos θ = 0 and no work is done.

With an understanding of work, we can get back to the main topic - Watts.  This is simply a measure of the rate at which work is carried out,  i.e.:

1 Watt = 1 Joule of work per Second  

What is the rate of doing work? The answer is Power.  If we want to measure the power of a car or electrical motor we want to look at the amount of energy it can deliver every second (i.e. Watts).    The unit Watt is named after the Scottish engineer James Watt (famous for his work on steam engines). 

With an understanding of the concepts,  an example will tie everything together:

Example:  a 50 kG weight is lifted vertically 20 m in 10 seconds (with the lifting force acting in the same direction as the movement):

Work done = 50  x 20 = 1,000 Joules

The work is done over 10 seconds, so the average power (rate of using energy) is:

Power = 1,000 / 10 = 100 Watts

That’s  it.  Before finishing a few final observations:

  • 1 kW (kilowatt) is 1000 W (Watts).   Watt is the SI unit or power, although in the United States horse power (HP) as a unit is still common.  One HP is nominally the amount of energy a horse could deliver each second.  As each real horse is different this has been standardised to the Watt, with  1 HP  = 764 Watts
  • kWHr (kilowatt-hours) is the power multiplied by the time it is used. For example if 1 kW is used for 2 hours than 2 kWHr is used  (or 2,000 W x 7,200 seconds = 14.4 10^6 Watt-Seconds). Form the above, 1 watt-second is 1 Joule (i..e the energy expended).  kWHr measures energy and not power


Steven McFadyen's avatar Steven McFadyen

Steven has over twenty five years experience working on some of the largest construction projects. He has a deep technical understanding of electrical engineering and is keen to share this knowledge. About the author

myElectrical Engineering

comments powered by Disqus



What are you reading!

Reading is a bit of a hobby of mine and I"ve done a few off-topic posts in the past on this. Rather than continue doing the occasional post I thought ...

110 or 230 Volts

I've been considering a blog on the 110 or 230 Volt issue for a while.  While browsing the Internet I came across a great summary by Borat over at  engineering...

IEC Reference Designations

The IEC publishes a series of documents and rules governing the preparation of documents, drawings and the referencing of equipment.   Depending on country...

Surface Treatment – Ladders, Trays and Baskets

Steel ladders, trays and baskets form the backbone of cable containment systems. Often these items need some form of surface treatment to prevent corrosion...

Multimeter

Multimeters are undoubtedly the most common item of electrical test equipment in use.  Often it is the first piece of equipment people will turn to when...

Understanding LV Circuit Breaker Fault Ratings

I think this post is going to be helpful to several of our readers. While the IEC low voltage circuit breaker Standard [IEC 60947-2, Low voltage switchgear...

Tip – Latitude and Longitude on Large Scale Plans

If you are working on a large plan, get the real coordinates [latitude, longitude] for two or more points and add them to the drawing. That way you can...

RLC Circuit, Resistor Power Loss - some Modelica experiments

Modelica is an open source (free) software language for modelling complex systems. Having never used it before, I thought I would download a development...

Tips for a better Low Voltage Protection Discrimination Study

Carrying out a protection system discrimination study is critical to ensure the correct functioning of  the electrical system in the event of faults. ...

Aluminium Windings - Dry Type Transformers

The other day I was talking to a colleague who is a building services consultant.  Despite regularly specifying dry-type/cast resin transformers he was...

Have some knowledge to share

If you have some expert knowledge or experience, why not consider sharing this with our community.  

By writing an electrical note, you will be educating our users and at the same time promoting your expertise within the engineering community.

To get started and understand our policy, you can read our How to Write an Electrical Note