The Kwh

A Watt is a unit of energy ( named after James Watt ) A Kilowatt is simply 1000 watts.

When you use a Kilowatt of energy for one hour, then this is a Kilowatt hour or Kwh.


For Electricity


All electrical appliances have a rating in Watts, such as a 100W (0 .1Kw) electric light bulb, or a 2 KW  heater.

If you were to use the electric light bulb for 10 hours, then you would have used –

                               0.1Kw  x 10 hours  = 1Kwh of electricity,


and if the price of your electricity is 12p per Kwh,  then it costs-   ( 1x  0. 12)  or 12p to run your bulb for 10 hours.


If you were to use the heater for 10 hours, then you would have used –


2Kw  x 10 hours  = 20Kwh of electricity,


which would cost  20 x .12 = £2.40  to run the heater for 10 hours.


Electric meters measure directly the amount of energy used in Kwh and so they are read in Kwh units.



For Gas


Gas is measured in cubic metres ( or cubic feet ) which is a measure of volume. There needs to be some way of establishing how much energy is contained within a volume of gas. This will depend on the quality of the gas, which can vary depending on the gas field from which it was delivered.

Natural gas can contain water vapour and other impurities, so samples are analysed to quantify the potential energy within the gas sample. This is a figure known as the calorific value.


Gases expand and contract with variations in temperature, and so the volume of gas in a sample will vary according to the weather conditions it is exposed to. Clearly an adjustment in whatever volume a gas meter reports is necessary to compensate. This is known as the Volume correction factor.


A formula is used to convert all this information to KWh, which is the estimated energy which has been delivered through the meter.


To convert Cubic Metres to Kwh, multiply the volume in cubic metres by the calorific value, then multiply the result by the volume correction factor. Divide the result by 3.6 to produce an answer in Kwh.


NB  - If the gas meter measures in the old imperial units of cubic feet, you will first need to multiply by 2.83 to convert to cubic metres.

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