Since Gordon Brown on Thursday launched what he called "the greatest revolution in our energy policy since the advent of nuclear power", centred on building thousands of new wind turbines, let us start with a simple fact.

Nothing conveys the futility of wind power more vividly than this: that all the electricity generated by the 2,000 wind turbines already built in Britain is still less than that produced by a single medium-sized conventional power station.

There are nearly 50 nuclear, gas or coal-fired power plants in Britain today each of which produces more electricity in a year than all those 2,000 turbines put together.

I make no apology for returning to this subject because the "£100 billion green energy strategy" published last week, by what is now laughably known as the Department for Business, Enterprise and Regulatory Reform (BERR), contains not only many smaller deceptions and self-deceptions but one so great that almost everyone has fallen for it.

The starting point is the EU's requirement that, to combat the "threat of climate change", we must drastically reduce our CO2 emissions, chiefly by building thousands more wind turbines.

It is quite clear from the paper that BERR's officials know we haven't the faintest hope of meeting our EU target in this way. So its number-crunchers have been working overtime to squeeze down the amount of energy we source from wind to the lowest figure it thinks can be made to sound plausible.

Until last week BERR had been claiming that our EU requirement meant that we must generate 38 per cent of our electricity from renewables, the largest contribution coming from 11,000 offshore turbines, representing 33 gigawatts (GW) of capacity. But all this has changed dramatically.

They now talk only about the need to meet 32 per cent of our total EU renewables target through our methods of electricity generation, with only 32 per cent of that needing to come from wind - and that, they say, can be done with a mere 7,000 new offshore and onshore turbines.

However, our present generating capacity is 76GW. By 2020, on projected demand, to replace one third of one third of our capacity with wind power would mean generating an average of 10GW.

And herein lies the central misconception which bedevils the entire debate. Because of the wind's intermittency, turbines generate on average at less than a third of their capacity. Thus to contribute 10GW would need 30GW of capacity, which would require up to twice as many turbines as ministers are talking about - needing to be erected at a rate of more than four every working day between now and 2020.

In practical terms, even if they grossly bend the planning rules (as MPs voted for last week), there isn't the remotest chance that anything like this number of turbines could be built in time to meet their target.

For instance, the world only has five of the giant barges that can install monster turbines offshore - and for more than half the year our weather conditions make installation impossible anyway.

But in addition we should also need to build at least 20 new conventional power stations simply to provide back-up for all the times when the wind is not blowing - at a time when, within seven years, we already stand to lose 40 per cent of our existing generating capacity through the closure of almost all our ageing nuclear power plants and half our major coal and oil-fired power stations (due to the crippling cost of complying with an EU anti-pollution directive).

It is a total mess. The reality is that, thanks to the dithering and wishful thinking of our politicians, it may already be too late to avert that breakdown of our electricity supply which would be one of the most serious disasters Britain has ever faced.

And, ironically, no one at present looks more likely to inherit this mess than David Cameron - whose only response to last week' s pie-in-the-sky from Gordon Brown was to say that the Government should have been building all those useless windmills years ago.