Energy units – muddle in the making?

It was decided early in the 1990s that the unit for pricing of domestic gas should change from the therm to the kilowatt hour, which is a metric unit but not SI. We ask if this made ‘metric sense’.

We recently came across an article in the December 2017 edition of The Yardstick, the newsletter of the British Weights and Measures Association (BWMA), which began:

“… BWMA’s John Strange has pointed out to the government that the EC had not replaced the British therm with a metric unit but with a non-metric unit known as the kWh, making nonsense of ‘metrication’ as a policy.”

This caused us to ask why the government chose the kWh, a unit that is not included in the international metric system (“Système International” – SI), rather than the SI unit for energy, the joule, and also to ask if this made sense.

But first we need to correct two errors made by Mr Strange:
1. The EC does not mandate the use of the kWh – it appears nowhere in the Units of Measurement directive.
2. The kWh is of course a metric unit, obtained by multiplying an SI derived unit, the kilowatt, by a unit accepted for use with SI, the hour. (It is also in practice an Imperial unit – when the Imperial system was formalised in 1824 electricity was still an experimental science and no electrical units were included.)

We can only guess why the government chose the kWh rather than the MJ for gas. The use of a unit that the public was familiar with would have eased the transition as the new gas prices could be easily compared with existing electricity prices. We think Anne Attlee’s Metric Sense campaign would have approved.

This choice was not inevitable. When visiting Melbourne in 2006, the author noticed that domestic energy, both gas and electricity, was priced per megajoule. And the UK Metric Association (UKMA) advocates the full adoption of SI for all official, trade, legal, contractual and other purposes in the United Kingdom as soon as practicable.

So, was the government right to go for a quick win, or has it, as is often the case, merely postpone a difficult decision? Readers’ views are welcome.

A recent comment on Metric Views drew attention to a different area where two metric units are in contention: the use of calories and joules to measure food energy. And in future, we will be looking at another potential kWh versus MJ battleground – vehicle fuel consumption. We shall be inviting owners of hybrid and electric cars to pass on their experiences.

The article referred to above may be found here:
BWMA web site
(The Yardstick 65, December 2017, page 10)

2 thoughts on “Energy units – muddle in the making?”

  1. And in schools and colleges more confusion for students:
    In Science including Physics I expect:
    kilowatt hours and joules to be used,
    in cooking/catering courses (what many decades ago – used to be called ‘Home Economics’) kilocalories are used.

    I’m not sure if NHS professionals are being encouraged to use kJ and MJ.
    Sadly I expect NHS Courses for staff – eg. nutrition, to reduce obesity etc. these courses only use kcal.

    In another blog article it was pointed out that in 1972 The Royal Society recommended only joules to be used.
    ‘Calorie intake’ shouldn’t be used; just use ‘Energy intake’.

    Wikipedia is one place where you’ll find some examples showing what one joule in everyday life represents, it also lists examples using multiples, for example:
    the thermal output of the Sun is approximately 400 YJ per second.


  2. A rather late reply to this thread I must admit. However I have just browsed a banal subject of how to read a gas meter and found it rather interesting about Joules v kWh.
    Step 3 and 5 are of interest
    step 3 includes “Calorific values vary and the figure quoted on your bill (eg 39.5 megajoules per cubic metre (MJ/m³)) will be an average of the gas supplied to your property (the regulations explain how this is calculated). Gas transporters are required to maintain this figure within 38 MJ/m³ to 41 MJ/m³ as figures outside of this range will cause problems with gas burning appliances.
    Step 5 then goes on
    “Finally the figure is converted to kWh by dividing by 3.6. Again, this factor is prescribed in the above regulations.”

    So, first why convert to kWh when it is already in joules, and second why is there so much apparent opposition to joules when it is firmly entrenched in the UK gas industry?


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