National Grid says that the country has the electricity generating capacity to meet the average maximum need over the course of the UK winter. But this calculation critically depends on the reliability of power stations as well as an accurate assessment of the true generating capacity of each plant. This article looks at National Grid’s assumptions on power station availability over the next months and casts a somewhat surprised eye on its apparent errors, particularly in calculating the likely output from nuclear stations. National Grid has raised its assessment of the nuclear fleet’s availability, and by more than any other major type of power station. It predicts that 90% of the UK nuclear capacity will be working at the point of maximum demand, up from 84% last year. In the face of repeated unplanned shut downs at EdF’s plants this year, I can think of absolutely no reason for this enhanced optimism. And, indeed, National Grid’s cheery forecast is not shared by Ofgem, which held its estimate at 81% availability, in its report in mid-summer. The Ofgem document actually predates the unplanned closures at Hartlepool and Heysham 1 that started a couple of months ago and I doubt Ofgem would be as optimistic today. I looked at the performance of the UK’s nuclear fleet from early December to mid-February this year. Only for a couple of days did it actually achieve the 90% output that National Grid – based on information from operator EdF – suggested it will for 2014/2015. Average performance was 81% of potential, in line with Ofgem’s more conservative forecasts for this winter and last. As I write this, only 3 of EdF’s nuclear generating units out of 16 (in eight power stations on seven sites) are working to their full rated capacity. A further 4 are operating at 20% below maximum power as a precaution. Sizewell (one station but two turbine units) is on a planned refuelling stop. 2 other units are suffering from mechanical faults and 4 are being inspected for a possible problem in their boiler units and will return to operation between now and the end of December, although at a lower output than previously. Another plant is returning to full power after refuelling.