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:

www.metrication.us/content/demise-mpg

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?