THE full cost of rewiring Scotland to exploit its green energy potential is now becoming much clearer. In January Scottish and Southern Energy revealed its preferred route for a new 400kV transmission line stretching 220km from Beauly in the Highlands to Denny near Stirling. If approved, that project alone will cost some £200m.

This week ScottishPower painted in details of what its main grid will look like in future, including upgrading a number of existing routes, installing three new substations each the size of two Hampden Parks and building a completely new 400kV transmission line stretching 60km from Kilmarnock, through South Ayrshire to Galloway. Phase one of this programme will cost £225m.

If enough windfarms are eventually built along Scotland’s western seaboard, many of them to satisfy the government’s renewables quota south of the Border, there could be a second phase, extending the high-voltage line. with pylons nearly 50m high, across southwest Scotland, passing just north of Dumfries and linking up with the English grid at Harker near Carlisle. The price tag for that phase is another £200m.

That’s more than £600m for the high-voltage grid wiring alone. Many more millions will doubtless have to be ploughed into upgrading links between where some of the major windfarm clusters are likely to be built (on offshore islands, for instance) and the main transmission spine which has been the focus of attention this far.

Then there’s the cost of the windfarms themselves. Judged by the price tags of recently approved projects, like SSE’s Hadyard Hill farm in South Ayrshire, it will cost a further £70m for each and every 100MW of installed windfarm capacity.

To hit the government’s target of 10% of UK energy consumption coming from renewables by 2010, we could be talking about 4000 megawatts of windfarm capacity coming on stream within the next six years. That would cost the thick end of £3bn.

And if the government means what it says about doubling that percentage to 20% by 2020, we could be looking at at least £6bn of investment in renewables by then. The Scottish Executive’s dream of 40% of Scottish electricity consumption being from green sources by 2020 could push our share of the costs even higher.

The cost of the upgraded Scottish grid, some £625m on current plans, is to be spread across the bills of all 25m electricity customers in the UK on the grounds that, while Scotland has a lot of the best wind, the pursuit of a low carbon economy is in the interests of all who live on these islands.

But eventually all this massive investment – some £7bn and rising in Scotland alone, if my analysis is anywhere near the mark – will have to be recouped. Even the government concedes that a lower carbon economy could put between 10% and 20% on the average electricity bill. Some believe that to be a serious underestimate.

It will certainly puncture any notion that harnessing abundant wind power now – or wave and tidal power hereafter – is going to be cheap, despite longstanding attempts by the green lobby to foster that myth.

If this particular penny were to drop with the general public now it could make the challenge of winning popular consent for large windfarms, sub stations the size of two Hampden Parks and major new power lines marching through some of the most sensitive parts of rural Scotland a great deal harder than it already is.

Power company executives know they have a tough job on their hands persuading people that a line of 47m-high pylons within sight of their windows is a price worth paying for saving the planet.

I was only half joking when I suggested to some of them this week that objectors might rally round the slogan “It’s England’s Pylons”, when the planning battles over these proposals really begin in earnest.

Some of these planned transmission lines are going to pass through areas populated by the retired middle classes, with the time, knowledge and ingenuity to mount challenges against the logic that lies behind these investment programmes.

The power companies are quick to stress that they are acting as agents of a Scottish Executive determined to pursue highly-ambitious renewables targets, both as Scotland’s contribution to combating climate change and as a way of breathing fresh life into Scotland’s shrunken manufacturing sector.

I can see retired diplomats and company executives in southwest Scotland, for instance, forensically unravelling the green logic of harnessing energy from fickle wind power in Skye, say, or on the Mull of Kintyre and then transporting it, with all the associated transmission losses, over an expensive new super-grid, past their front doors, all the way to the English Midlands.

They may even find some inspiration in recent counterblasts from that noted green scientist Professor James Lovelock, who has described renewable energy as “a showy way for politicians to prove they are doing something” about global warming.

Lovelock has scandalised fellow-greens by suggesting that “clean and safe nuclear energy” might have a major role to play in our future energy mix. “To supplement the feeble energy supplies from renewables by lashings of natural gas is a risky option, particularly for Britain,” he argues, especially when that “tried and tested” alternative is to hand.

But that’s precisely where we are. Government and the power utilities are largely silent on what happens when our existing coal-fired and nuclear baseload generating capacity reaches the end of its useful life over the next 15 to 20 years.

They may find it impossible to maintain that silence as they try to persuade us all that rewiring Scotland is vital if we are to save the planet from our own profligacy.