You need the brainpower of a supercomputer to work out whether an electric car would save you money or really help the planet turn greener writes Ian Donaldson.

For starters, they’re not cheap and may not even be all that climate friendly to run if the electricity they need comes from a coal-fired power station, belching out carbon dioxide like there’s no tomorrow.

Even so, there’s a growing market for cars that add a green sheen to the business of motoring, In the right circumstances they might even start to make sense.

Whatever the accountants might say, there’s no stopping the car makers in their rush to turn green and the Vauxhall Ampera is simply the latest in a growing line of cars with boosted battery power.

It might, however, be the most sensible one so far.

There are basically three ways of giving a car some electrical eco-cred. The obvious one is to replace the petrol or diesel engine with a great big battery and be done with it.

The trouble is that you’ll forever be worrying about the car’s range. Fear of getting stuck miles from home in the dark, with the battery flat and needing an eight hour charge has helped coin a new motoring term: range anxiety.

The second way to burnish your eco-halo is to go the Toyota Prius route, using a conventional petrol engine most of the time but backed by a biggish battery that helps out when it can.

Most of the time you’ll be on petrol power, but more than two million owners worldwide have succumbed to the charms of a Prius.

Then there’s a third way, as found in the car featured here. The Ampera has a huge battery (actually, lots of smaller packs running safely out of harm’s way down the spine of the car) which will take it up to 50 miles on a good day before a modest 1.4 litre petrol engine from an Astra takes over.

That way, says Vauxhall, most Amperas won’t ever need to turn their petrol engines on because most of us drive no more than 40 miles a day.

So, no range anxiety (you can fill the Ampera’s petrol tank like any other car, after all) and no road tax or congestion charges and a modest five per cent hit on company car tax. Oh, and an electric fill up takes four hours from your home supply and costs about £1.

It even looks like a rather smart conventional car (it’s built in the States and sales here are forecast as modest) with a comfy interior, decent boot and nothing apart from its badges to denote you as a green eco warrior.

Push the go-faster pedal to the floor and the Astra-sized Ampera scurries off with just a bit of tyre noise and reaches 30mph as quickly as a hot hatch.

It will touch 60mph in around nine seconds (a respectable figure) but is limited to 100mph to save battery power.

Which is another way of saying that slower is much better with the Ampera, if you want to go electric for as long as possible.

Driving normally on the press introduction (around a very flat Amsterdam) my car ran out of current at 38 miles and the petrol motor kicked in seamlessly to take over.

An impressive performance, I thought, but you need to read on before placing your order for an Ampera, due here in early 2012.

It might use cheap electricity but the car will cost £28,995 and that’s after a £5,000 government grant has been deducted.

There are an awful lot of fine conventionally powered cars available at that price, so you’ll need some eco fire in your belly to favour an Ampera.

Longer term, running costs should beat a more ordinary machine and there’s an eight year or 100,000 mile warranty on the battery.

And, unlike owners of a purely electrical car, you won’t need a second vehicle on standby for longer runs.

Even so, it looks an expensive way to save money. But if you’re tempted and can afford it, the Ampera makes a convincing move into the future.

Probably.