Electric cars – an opportunity for SI, or a threat?

The arrival of electric cars on our streets will draw attention to measures for comparing performance.

Go into your local DIY superstore, and head for the lawn mowers. Now try to compare the power of different models. For small petrol mowers it is likely to be given in cc, for large petrol mowers in hp, or PS for German models; electric mowers will be rated in W or kW.

Of course, it is rarely necessary to compare the power output of different types of mowers, because the type required is usually determined by the lawn to be mown. However, for cars the choice is not so simple, and anyone considering the purchase of an electric car will surely wish to evaluate its performance against that of comparable petrol or diesel models.

Tim Bentley, a frequent contributor to MetricViews, writes:

“With a huge growth in the number of electric cars about to be launched on the British market, it is now time to adopt the kW as the standard unit of power for all cars. Whilst kW is generally used for electric cars, hp, bhp and PS are used for petrol and diesel cars. It is important that customers are able to compare the different cars on offer and the use of a standard unit (kW) is not only sensible but essential.”

A webpage from ‘Which’ highlights this problem: www.which.co.uk/advice/power-converter/index.jsp

The arrival of electric cars also provides scope for confusion in the matter of fuel consumption. A useful article about this appears on the US site Metrication.US:


David Brown, well known to readers of MetricViews, provides a helpful comment on the US web site.

The US article understandably omits to mention the confusion arising from the difference between the US gallon and the imperial gallon, which is still used alongside L/100 km for fuel consumption in the UK.

However, if both the British Government and the shadow transport secretary don’t want road users to get their heads round km for distance and km/h for speed, as appears to be the case, then what chance is there for MJ/km for consumption of fuel (or energy) and p/MJ for price?

12 thoughts on “Electric cars – an opportunity for SI, or a threat?”

  1. So far from what I have read in regards to electric vehicles, miles, MPG psi, lbf and bhp are still being used to describe them. I watched a video of the unveiling of a track-only electric motorcycle and the power rating was given in hp in the announcement.

    I emailed Honda in regards to updating their motorcycle pages as the layout has not been changed in years and I did commend them for using only metric units in the specifications in the hope that this show them that this is worth continuing in the future. However their car pages are a mix of units, not even stating power in kW.

    I believe the mess is going to carry forward to electric vehicles. I think in due part to the UK and US markets.


  2. SI is always a threat to those who market products and services. They want units of measure that are difficult to understand and different units for different applications. This way comparison is either difficult or impossible and most consumers will shun comparing products on a measurement basis. If units were used that would allow easy comparisons of products and prices then it would become impossible for marketers to scam the consumers with illusions that their product is a better value then a competitor.

    I’m sure the kilowatt will become the standard for electric cars and the old horsepower in its various forms will remain for non-electric vehicles as a means to make comparisons between the two types very difficult. The horsepower is preferred in some circles because it is meaningless to the average consumer (ask anyone who uses the term horsepower what it means to them and the response will be vague) and because the numbers appear larger the the kilowatt, giving the impression of getting more.

    Even in the UK with the use of the litre to purchase petrol, the mpg is still insisted upon, simply because the calculation becomes difficult and it is intended that the average person not bother to figure out how much their vehicles are really using.

    As long as marketers continue to misrepresent their products, they will always push to use a collection of confusing units, unless they are forced to use standard SI units by law.


  3. I was reading only the other day about a hybrid car, in which both the petrol engine AND the electric motor ratings were given in hp. I can, to a small extent, undertand the petrol engine being rated in hp, but the electric motor? Bizarre. I can only assume that it was to provide a comparison between the two forms of propulsion – in which case, it surely makes more sense to convert the petrol engine to kW rather than the other way round. As this Metric Views article states, the mess looks likely to continue – but why?


  4. The reason for choosing the horsepower over the kilowatt is quite simple. It is all a part of deceptive advertising. The horsepower provides the illusion that one is getting more power, because the numbers are bigger. 50 kW of power is about 67 horsepower. 67 is a bigger number then 50 and gives the impression one is getting more power by stating the value in horse power.

    The horsepower is also a unit that can’t be related easily to something else. Thus the average person has no real reference points for understanding how much power a horsepower really is. A kilowatt however has a number of common reference points. It is easy to image the kilowatt as the power to light 10 x 100 W light bulbs. The kilowatt is also used to measure the output of electric heaters as well as countless other home products. Thus the kilowatt is easy to reference.

    By not having a suitable reference for the horsepower, the use of it makes a very handy means to deceive and deception is what it is all about.


  5. How sad and baffling that the PM refers to persons who deny climate change science as “flat earthers” and “anti-science”, yet he does not apparently see that clinging to an outmode hodge podge of measurement units (Imperial “system”) is very much the same.

    If he did and pushed hard for it, the UK could go nearly 100% metric by the time of the London Olympics. Maybe he’s just waiting for the results of the next election??


  6. Jeremiah is spot on with his comments on marketing. Whatever ways can be found of getting around fair comparisons will be employed. To digress slightly, when I lived in the UK I noted that coffee was a good example of this. It would be sold in UK-packaged 227g packs and imported (usually Italian) 250g packs. The headline price would be lower for the UK-packaged product. Needless to say, the unit price (in a very small font) would show a different story.

    As for petrol, the “mpg” figure seems bigger, therefore better. It is interesting to note, however, than in everyday parlance, many countries prefer using km/l rather than the official l/100 km. My point still holds though.


  7. Whilst the above remarks about marketing are true it isn’t the only reason.

    People think the watt is an exclusively electrical unit like the volt. It’s poor understanding of basic science.


  8. Richard Birkby wrote “As for petrol, the “mpg” figure seems bigger, therefore better.” This is purely a result of cultural compartmentalising. People need to be educated that costs can be expressed not just in money, but also in commodities such as petrol.

    Thus, bananas at 88p/kg cost less than bananas at £1/kg. It should be the same with petrol – If I “pay” 8 litres of petrol to travel 100 km, then I am “paying” more than if I “pay” 6 litres to travel the same distance. Given the current Copenhaegn conference, this manner of thinking is particularly topical.


  9. The term ‘mpg’ is often referred to as consumption when in fact it is a measure of economy.
    L/100 km measures this directly and is on the whole a better way of doing so. It makes both types of calculation relatively easy i.e. how much fuel will be used on a given journey, or how far will a vehicle travel on a given amount of fuel.
    That’s probably the reason for the convention of L/100 km rather than L/km. It accomodates the second type of calculation more easily without losing the simplicity of the first.


  10. Martin Vlietstra is quite right – education (and public awareness) is a key issue. He echoes and reinforces philh’s comments by his example.


  11. As a side issue to the use of electric or hybrid cars, at the recent climate conference in Copenhagen, there were over 1200 vehicles provided to move delegates around, including to and from the airport. Precisely five of these – the fingers on one hand – were ‘alternative’ propulsion’ – i.e. not petrol or diesel, but either pure electric or hybrid.

    We measure vehicle CO2 emissions, even in the backwards UK, as g/km. The USA measures its vehicle emissions in g/mile. As we are all being asked to become aware of our carbon footprint, it would be nice to see some universal (metric) standards as to how this should be measured. And then get the media, including the BBC and the popular press, to use it, instead of simply saying ‘reduce your carbon footprint’ but giving no criteria as to how we measure any reductions.


  12. A few years back I was sent an email of a video of one of Al Gore’s seminars on Global warming. Even though all of his visual displays used metric units, Al spoke mostly in USC. It was very confusing to compare what Al was saying to what was on the displays. I wonder if the people in the audience felt the same way, especially those who may have attended and didn’t have a clue to what the units Al spoke meant.

    In addition when I hear someone giving a speech on a scientific subject and does not use metric units, I don’t take their point seriously and view it as pseudo-science. If they can’t use proper units of measure then to me their doctrine can’t be sound.


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