Toyota and energy company EDF today (10 September) announced plans to trial plug-in electric hybrid vehicles in the UK but, interestingly, the trial is not so much about the technology as understanding the way that people use their cars.

One of the problems with electric cars is that they have a limited range (because of battery limitations) and most vehicle manufacturers have been working on increasing this so that it is more comparable with conventional engines and thus more acceptable to consumers.

But Toyota's new plug-in version of the Prius will only travel 10km (six miles) on a single charge. Toyota reckons that most journeys are short and that owners would prefer to charge their car for two hours at a time between short journeys, rather than have to charge for eight hours overnight, for 100km (60 miles) range, for example.

A similar strategy is being adopted by GM for the Chevrolet Volt, due to go on sale in 2010. The Volt - production-ready photos were leaked on the internet this week - became feasible when executives recognised that average daily distances travelled in the US were just 53km (about 31 miles). So the Volt has been designed to run 60km (about 35 miles) on a single overnight charge.

It is important to point out that the Toyota/EDF trial announced today is taking place within the London area (inside the M25 London orbital motorway). A 10km range may be fine for London, which already has the lowest car ownership rates in the country, but what about the rest of the country? A local solution maybe, but at least Toyota cannot be accused of 'over-engineering'.

So are these cars green? According to EDF, by using electricity instead of petrol, the amount of carbon emitted by cars in the UK can be reduced by 40%, even with a relatively low proportion of nuclear and renewables in the electric power mix. Further carbon reductions can be achieved for miles driven in pure electric mode. In France, where trials are already underway, Toyota has found that on journeys below 25km the plug-in version of the Prius delivers 60% fewer emissions compared to the standard Prius.

One aspect of the Toyota/EDF trial in the UK is that some of the 50 cars will be tested over the next year with people who generate their own electricity at home using solar panels, for example. This could be a much quicker route to achieving carbon-neutral driving than waiting for energy companies to get their act together.

Sue Brown

See also: UK: Toyota starts plug-in hybrid trial