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The latest element of the UK government's low-carbon energy policy has just emerged; .

Drivers are to be offered cash incentives of up to Β£5,000 to trade in their petrol- or diesel-burning cars for new electric or plug-in hybrid models: not the models available today, but the ones that are said to offer a driving experience and utility comparable to today's fossil-fuelled vehicles.

This pounds-for-petrol trade is part of the UK's response to an issue that many other countries (and in the US, many states) are grappling with: how to take the carbon out of road transport.

Putting it crudely, three major technologies have been on the table: biofuels, hydrogen and electricity.

Hydrogen still looks costly and technically difficult, despite , and ; but battery technology is advancing so fast that that the electric car now appears to have overtaken its rivals as a potential mass-market "green" solution.

But - as has often been pointed out - electric cars (and electric hybrids) do not necessarily offer low-carbon benefits; it depends on how the electricity is generated, and to a lesser extent on the efficiency of the pathway that takes it down the wires from generator to battery, and the efficiency of the vehicle.

So how "green" are they?

With all these caveats, hard numbers are hard to come by. comes from in the US, which compared efficiencies and greenhouse gas emissions for 16 drivetrain permutations involving fuels such as petrol, diesel, ethanol and compressed natural gas, and power options such as internal combustion engines, hybrids and fuel cells.

The conclusions are far from clear-cut. Ethanol outperformed every other fuel on emissions, however it was used - but the study was done before some of the potentially damaging impacts of ethanol production all came to light.

Hybrids performed just a little better than conventional vehicles.

But any overall carbon savings hinge on the fuel mix of that particular country's electricity generation. The US proportionately uses more coal and less natural gas than the UK - so if the same study were done in the UK, it might project higher greenhouse gas reductions.

The highest savings of all might pertain in a country such as Iceland where virtually all electricity comes from renewable sources; France, with more than two thirds from nuclear, would also presumably fare relatively well.

Already it's clear, I hope, that the success or failure of an electric vehicle programme in reducing emissions hinges on a lot more than getting the fossil fuel burners off the road.

Let's make a leap of faith and say that the UK does switch successfully to a low-carbon electricity mix, through ramping up renewables and nuclear .

There now comes a problem. Many renewable technologies cannot generate on demand, and nuclear reactors like being on all the time; so you will generate too much electricity at some times of day, and too little at others.

Car batteries can be part of the solution. As proposed by the Californian company - and endorsed by a number of governments including Denmark's and Israel's - batteries can be the storage devices, the demand balancers.

When they're not being used in vehicles, they're connected to the grid, recharging when the system is awash with spare joules, and supplying energy at times of peak demand.

This kind of approach potentially reduces the price of electric motoring, because (rather like storage heaters) you'll "buy" electricity when it's cheap - and if it's taken up on a big enough scale, it facilitates the use of a higher proportion of nuclear and renewables in the electricity mix.

Encouraging such a transformation, of course, might require more planning and foresight than some governments traditionally give to their climate policies; nevertheless, it's a vision that potentially addresses several key problems in one go.

So far, so good. But perhaps a cautionary note is needed.

The UK's recently outlined .

Transport accounts for about 28% of the whole, and road transport for about 85% of that quarter.

So decarbonising the entire fuel cycle for the entire road transport sector would lop off less than one quarter of the nation's emissions; and that scale of decarbonisation is extremely unlikely, with the electricity mix still dominated by fossil fuels, and the growth in road transport emissions down to lorries and vans, for which fewer low-carbon options exist than for cars.

Although the committee concluded that a "major role is possible by 2020" for electric vehicles, the potential for reducing transport emissions "is dominated by the scope for improving fuel efficiency" of conventional vehicles.

Far less sexy than a silent, slinky electric coupe, I know; but perhaps the boring road could be the one most travelled.

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