Time to take a tilt at turbines.

First a disclaimer: Donald Trump is so much not my best friend that the last time I had a pop in print at his beloved golf complex, several US newspapers gleefully printed chunks of it.

Mr Trump's tantrum last week about wind turbines in general and the proposed offshore windfarm near the said golf complex in particular attracted well-deserved derision. It was both absurdly overblown and nakedly self-serving but that doesn't mean that every word of it was a lie. So here is today's proposition: "Taxing your citizens to subsidise wind projects owned by foreign energy companies will destroy your country and its economy" (Donald Trump to Alex Salmond). Agree or disagree.

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Frankly, I don't know. The problem is that nobody really knows, including the Scottish and UK governments. There's vague talk of £70 billion being needed to expand British offshore wind capacity. Because there are huge subsidies on offer, the argument is clouded by commercial and political interests. The voices you don't hear are those of engineers who have spent their lives with organisations like the National Grid keeping our lights on and those who understand the economics of total power system costing.

Onshore many of the best sites have been taken and there is an increasingly vocal lobby against both the turbines and the huge pylons required to transport the power. So, though it is currently about twice the cost of the onshore variety, offshore wind and undersea cables are attracting a buzz of enthusiasm. But before going ahead hell-for-leather it must be measured against three yardsticks: security of supply, carbon reduction and affordability.

On the first count, it's pretty hopeless. Wind can't be used to meet peak supply because sometimes it doesn't blow (or blows too hard). So we have to pay coal and gas-fired stations, running at 60% capacity, to fill the gaps. In 2010, on the four days of highest demand, the wind barely blew and, on December 8, Britain came perilously close to losing supply.

For the same reason, any CO2 savings are trivial, even if you don't count ultimate dismantling and disposal. Hydro could compensate for wind variability but there isn't enough of it. Once the genuinely low carbon nuclear stations are gone, that means relying on coal and gas generators. Because these use relatively more fuel when used as back-up than at full capacity, carbon savings are limited. That's why, over time and on a large scale, gas turbines alone may produce fewer emissions than windpower.

Finally, affordability. Lots of figures are bandied about but those in the know say we need a predictive model using "levelised costs". This assesses costs to consumers over the investment period. Including storage, back-up, demand management issues and extra transmission costs, onshore wind seems to cost about three times more than nuclear, coal, oil and gas. Offshore is around four times more. And, while oil and coal may eventually run low, shale gas (including loads in central Scotland) is coming whether we like it or not and it will render the economics of renewables even tougher.

Alex Salmond talks blithely of an independent Scotland exporting its excess windpower to England. But how can you sell what you can't predict and why should they pay more than their own marginal costs, especially if cheap French nuclear is nearer the epicentre of need?

Mr Salmond clearly hopes that if he repeats his "Nuclear bad, windpower good" mantra often enough, we will all joyfully jump "on board", as he puts it. There's now a noisy and powerful commercial lobby pressuring the Government to preserve high subsidies and get thousands of turbines built. For goodness sake, put this revolution on hold and commission the National Grid to conduct an independent engineering and mathematics-based study. We need proper cost benefit analysis and predictive modelling from the experts. Maybe offshore wind is the future but maybe Mr Trump is right. Who would invest in an independent Scotland with sky high energy costs?