What is a Watt?

Energy and Power

A lot of people get confused about the difference betwen energy and power. Power is the amount of energy per unit time. Since the proper unit of energy is the joule (J) then the unit of power is joules per second (j/s). This is called the watt(W).

However, many other units are fequently used for energy and this can lead to quite a bit of confusion.

If you are not familiar with unit prefixes such as Kilo and Giga see here.

Kilowatt Hours (KWhr)

Often energy is measured in Kilowatt Hours (KWhr). This is one watt being used (or produced) for one hour. Since there are 60×60 seconds in an hour 1Whr = 3600J and 1KWhr = 3,600,000J

Probably the best way to explain it is via a few examples.

Example 1

A computer uses 100 watts and is left on 24 hours a day. How much energy does it use per day?

100W for 24hrs uses 24 x 100 Whr = 2,400 Whr = 2.4KWhr

To get the answer in joules then we just have to remember that one watt is one joule per second. Therefore we just have to multiply by the number of seconds in 24 hours (60x60x24).

Therefore the answer in joules is

100W x (60 x 60 x 24) = 100 x 86400 = 8,640,000 joules = 8.64×106 J =  8.64 Megajoules (MJ).

We can just do a quick dimensional analysis to check our result. We have multiplied joule per second (j/s) by seconds (s). The seconds cancel so we are left with the answer is joules (j)

Example 2

It takes 2 minutes to boil 500ml of water using a 1.8KW kettle. How much energy was used?

2 minutes is 2/60 hours. Therefore the amount of energy used is 1.8×2/60 = 0.06KWhr which is 60Whrs.

To calculate the answer in joules:

A watt is one joule for one second. 2 minutes is 120 seconds therefore the number of joules is 1800×120 = 216,000J = 216KJ


To make things a bit more confusing there are two types of calorie. One is the small calorie (cal) and the other the large calorie or food calorie (Cal). One large calorie is 1000 small calorie i.e. 1Cal = 1Kcal.

The calorie (cal) is the amount of energy needed to heat 1ml of water by 1°C. This is about 4.2joules.

Btu (British Thermal Unit)

This is what is often used to calculate your gas bill – you pay for the energy from your gas not the actual amount of gas used.

1Btu is about 1055joules.

Electron Volt

This is a unit that is often used in nuclear physics. It is the energy gained by a single electron moved across a potential difference of one volt. It is about 1.6×10-19joules.

Other Units

There are lots of different energy units. These include mtoe (millions of tonne of oil equivalent) which is used in the oil industry. GWdays (Gigawatt days) which is used by the nuclear industry (note that this refers to the total energy output rather than just the electrical energy output).

To convert from these units try http://www.onlineconversion.com/energy.htm

What Unit To Use?

It does not really matter as long as you use the same units. If you are looking at data that uses different units then pick one unit (I always use joules) and convert the data into this unit.


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