When I was a child, we had a gramophone record of the musical story Peter and the Wolf, by Sergei Prokofiev.

The story begins with “My Dear Children . . .” and rolls out how Peter, a duck, a bird, Grandfather, a cat, a wolf and some huntsman feature in a story. It is a gentle musical tale and I love it very much. But it is a story.

Now I find myself, once more, writing about wind turbines and I feel I would love to send a letter to all the banks and financiers of wind-turbine projects. The letter would begin: “My Dear Children . . .”, and would go on as follows: “Many of you are offering finance for the construction of wind-turbine farms. No doubt, you are satisfied that wind farm owners are going to make a good bundle of money and thus, if the grace of heaven is upon you, so will you banks and financiers.

“Now, oddly enough, when Terry Mackenzie-Hoy goes to get finance for a vehicle, he has to produce a whole lot of stuff proving he can pay the loan back – salary slips, proof of land ownership, and so on – and the loan is granted. Should ol’ Terry default on payments you can get some of your money back as fast as you can say ‘repo man’ – so, the loan is secure, sort of.

“Now Innowind, proposer of a number of wind farms, produced a natty little expense and income sheet, which showed that the internal rate of return on an investment in various wind farms is really very good – about 30%. The problem is, now that the air becomes black with wolves emerging from the snow, the calculations of Innowind and others are pretty much wrong. We all know that the wind does not blow all the time and thus all calculations assume a certain ‘capacity factor’, which is the ratio of the actual output of a wind turbine over a period to its potential output if it were possible for it to operate at full load all the time. Innowind et al give factors of about 30%.

“However, in a paper published by Nicolas Boccard, of the University of Girona, in Spain, studies of existing installations show that capacity factors are generally much lower than this – about 21%. What this means is that income (and thus repayments) from a wind turbine operator can be 9% less than hoped for. But it does get worse. He concludes: “The capacity factor is a crucial information for decision-makers and the reliance on the now popular (30% to 35%) value is not without consequences. For the private investor, the net present return of a wind turbine is proportional to its average capacity factor over the 20-year lifetime of the equipment.

“The average cost of a wind turbine installation is the ratio of fixed cost to capacity factor, up to a constant (8 760, the number of hours in a year). Since the ratio of predicted to real capacity factor is about 30:21, the real cost is 42% above the estimate.

“Now the worry- ing thing, the real beetle in the Christmas tree, is that Boccard is a big supporter of wind energy. He is not writing this to shoot it down – he is writing it to warn people to select good sites for wind turbines or lose their boots. And he wrote it in . . . 2008, which is interesting.

“How is it that all the wind turbine suppliers in South Africa seem to ignore the research, which was done five years ago? Well, it may just be that they have all chosen sites with more wind than the ones studied by Boccard. In point of fact, no. The sites chosen by Boccard are from all over Europe and the US and there are lots of them. So he’s correct. Thus, My Dear, Dear Children, your investors in wind power may well turn out to be Not A Terribly Good Idea. A bit like Peter walking in the forest when wolves are around and just hoping that the hunters will pitch up to save him . . .”