In his response to the article on wind power written by Mark Lynas and me, Professor Gordon Hughes says that gas turbines need to be kept running because the amount of electricity generated by wind varies so rapidly. This short note examines the actual variability of wind power generation over the last three months and compares it to the variability of total demand for electricity. I show that the demand for power is typically over ten times as variable as the supply of wind generated electricity. The point is this: if the National Grid can cope with large half hourly swings in the demand for electricity, then it can cope with the erratic supply from wind farms. Because supply and demand must balance on an electricity grid, swings in demand have exactly the same impact as similarly sized variations in supply. I analysed the electricity produced each half hour from 2nd July to today, 2nd October.

Degree of variation between one half hour and the next	Number of instances 2^nd July to 2^nd October

Less than 50 MW	2698
51-100 MW	1087
101-200 MW	550
201-300 MW	91
301-400 MW	11
401-500 MW	2
501-600 MW	1
601-700 MW	1

Average variation	52 MW

The average variation in wind output was 52 MW. I then compared this figure to swings in total electricity demand in the same period. The average variation in demand was 678 MW, more than ten times as great as the average variability of wind output. In fact, the maximum variation in wind output between adjacent half hours (674 MW) was less than the average variation in total demand. The maximum half hourly swing in demand was almost four gigawatts, or about six times the maximum variation in wind power output.

The National Grid can cope with much more rapid changes in the supply or demand for electricity than are currently ever likely from the use of the current number of wind farms.

Some criticisms can be made of my simple comparison. First, I am using data from a time of year when wind power generation is relatively low. In winter, variability of wind generation will be greater. But variations in demand will also be much greater in the dark months of December and January. Second, it can be contended that demand variations are more predictable than swings in wind power. This point has some validity: demand moves up and down each day according to a relatively predictable pattern. However unforecast variations from the predicted level of demand can and do occur and these will be far greater than today's wind variability. Weather forecasting allows good prediction of when power will increase or drop. Third, what is true today may not be true when the UK grid has to cope with perhaps five times as much wind power as at present. However even if we multiply the maximum half hourly variability of wind power in the last three months five-fold, we would still see less variation in supply than the maximum variation in demand experienced over the last three months.

Wind does not impose on the National Grid a substantial extra burden to balance supply and demand than exists already.