ALAN Rayner is correct in his letter ("Justify Array homes figure", Journal, September 26): the number of homes that could be supplied with electricity by the Atlantic Array is calculated from the capacity factor. He is also correct in asserting that the industry figure is 30 per cent or lower.
He then goes on to speculate on a figure of 650,000 homes supplied by the Atlantic Array rather than the 900,000 figure quoted by RWE. So who is right? Let us see if we can get to the facts behind these numbers.
First, we need to consult DUKES, the Digest of UK Energy Statistics. In DUKES 2013, Table 5D shows the electricity sales figures for 2011. From this we can calculate that the average household in the UK consumes 4,078 units of electricity in the year and the average South West household slightly more at 4,318 units.
The capacity factor is just the average output from a wind farm. In this instance you would calculate it by multiplying the number of households by the household consumption and then dividing by the number of hours in a year and dividing again by the maximum generating capacity of the wind farm which is 1.2 million units per hour (1,200 megawatts). This will give us the capacity factor as a decimal fraction. Multiply this by 100 and you will have the capacity factor (or average output) as a percentage.
The answers that we get according to Mr Rayner are an average of 25.2 per cent if he is talking about UK households and 26.7 per cent for South West households. The corresponding figures for RWE are 34.9 per cent and 36.9 per cent respectively. So who is right?
Well, if we are talking about the average onshore wind farm then Mr Rayner would be about right. But we are not. We are talking about an offshore wind farm. Most of the UK's offshore wind farms have capacity factors in the high thirties.
So, bearing in mind that Atlantic Array would have a better exposure to the prevailing south-westerly winds than any existing wind farm in the UK, on balance it would seem that RWE are correct and might possibly be understating the case.
North Devon Green Party.