Better average outputs will mean UK wind will frequently meet entire national electricity needs

A new analysis shows that Britain’s wind farms are expected to get much more efficient. In recent years, the typical wind farm has produced about 32.4% of the maximum output. This is projected to rise to 39.4% in the next twenty years, a rise of over 20%. The increase comes from taller towers, bigger turbines and, most importantly, an increased number of offshore wind farms, which benefit from much higher average winds.

The recent paper by Iain Staffell at Imperial College and Stefan Pfenninger at ETH in Zurich uses a new method of forecasting turbine outputs called Reanalysis. This technique utilises historical atmospheric pressure data from NASA and other sources to estimate wind speeds at high resolution. Based on estimates of past wind speeds, the authors then forecast how much electricity the wind farms planned to be build around Europe will generate. The results have been checked by comparing them to the actual output achieved by existing wind farms.

The improvement in UK wind farm outputs are matched by increases in other countries. Most importantly, Germany is expected to see an increase from 19.5% efficiency now to over 29% in 2035. This huge rise comes from the rapid shift of new wind farm construction into the Baltic and North Seas. The average efficiency (often called the ‘capacity factor’) across Europe is projected to grow by nearly a third from 24.2% to 31.3%.

Staffell and Pfenninger’s paper provides a similar, but slightly higher, figure to the recent report from the ECIU think tank, which projected that average UK wind farms would achieve a capacity factor of 33% onshore and 40% offshore by 2030, thus averaging perhaps 37%.

The supporting data and software tools will be extraordinarily valuable to those groups, such as grid operators, looking at the likely impact of growing amounts of wind power.

In the main body of this article I use the research results to roughly predict how often wind power will cover all the UK’s needs by 2035, displacing all other forms of generation, including nuclear. This is an amateur example of how the Staffell and Pfenninger tools can be used.

What do the results mean for the UK?

Staffell and Pfenninger have counted the capacity of new wind farms now under construction or at some point in the UK planning process. They indicate that within twenty years the country could have up to 42.3 gigawatts (GW) of turbines. (The figure today is about 13 GW, including those not connected to the main transmission grid).

42.3 GW working at a capacity factor of 39.4% will provide about 146 Terawatt hours (TWh) of electricity. This is about 40% of the UK’s total need at present. National demand has been generally falling in recent years as a result of energy efficiency. This may continue, particularly as LED lighting replaces halogens and other types of bulbs. But new demands for power for charging cars and heating homes using heat pumps may stabilise the downward trend and will, in all probability, cause power needs to start to rise by the middle of the next decade. But 146 TWh will still provide a large fraction of total national requirements.

More specifically, what does greater wind output imply for other sources of electricity generation in the UK?

The electricity generated varies from almost nothing up to a maximum of about 90% of the rated capacity of wind farms. To some extent, the swings are predictable. We know that atmospheric conditions can mean one storm after another charging in from the Atlantic separated by four or five days. We also recognise that winter wind speeds are higher than those in July. Late autumn is surprisingly good. However conditions still vary dramatically from week to week, a fact that opponents of wind turbines focus upon.

Staffell and Pfenninger’s paper provides some extremely valuable new data on the daily and monthly variability of wind in the UK and other countries. It shows, for example, that typical wind speeds are roughly the same across all 24 hours in winter but that summer months see a peak in late afternoon.  (All their research is now freely available online, along with their modelling tools. I cannot stress enough how useful this will be to researchers and policy planners).

In the work below. I use their estimates of the capacity factor achieved by UK wind farms during the windiest 5% of the time. At the moment, this figure is 68% of maximum capacity. (Put another way, for five percent of the time each year, UK wind farms are producing at least 68% of their rated maximum output).

I have used, of course, different figures for each season for capacity factors because it is windier in winter and autumn. Winter is assumed in my rough analysis to see a capacity factor of 80% for the windiest 5% of the time in 2035, autumn is 75%, spring is 60% and summer 50%. These numbers are guesses but based on the averages in the Staffell/Pfenninger paper. They are unlikely to be significantly wrong. (Seasons are Months 12,1,2, Months 3,4,5, Months 6,7,8 and Months 9, 10 and 11).

In the remainder of this article I use their figures to make a rough estimate of how much of the time wind power in 2035 would fully cover today’s needs. I have had to make some guesses in my analysis, but a researcher devoting time and using the online resources would be able to make a clear estimate of the number of hours that wind will completely meet all UK requirements.

My result shows that in autumn and winter wind power is likely to be greater than national need on a substantial number of occasions. Every night in October 2015, for example, had total UK demand less than would have been provided by 42.3 GW of wind power on the windiest 5% of autumn 24 hour periods. Summer will see some half hours when wind exceeds demand however spring will see a surplus very infrequently indeed.

Why am I writing this article now? Because Staffell and Pfenninger’s work shows that some of the time the UK will have excess power and therefore needs to work harder to develop long-term energy storage able to take weeks of surplus electricity. Long term or ‘seasonal’ storage must move to the front of the research agenda.

And, second, if storage capability is not developed, Hinkley Point C will simply not be needed for substantial amounts of time from November to February. And this is before thinking about solar power (providing about 4% of UK electricity already), hydro, anaerobic digestion and other renewable sources such as the new tidal power farms in Scotland. The growth of intermittent renewables will eventually mean that the UK has too much power at times of high wind and sun to be able to cope with highly inflexible large-scale nuclear.

I have tried to express this as best I can in the following charts with the prospective wind output superimposed over the total UK demand for electricity every half hour from August 2015 to July 2016. (I have added in National Grid estimated figures for wind power not attached to the main grid, as well as estimated solar PV output).

Chart 1. The pattern of GB electricity demand (gigawatts)

Chart shows seasonal rise and fall as well as daily swings and differences between night and day, with summer weekend nights showing the lowest demand.

Chart shows seasonal rise and fall as well as daily swings and differences between night and day, with summer weekend nights showing the lowest demand.

Total demand peaked at around 52 GW in the latter part of January 2016. The lowest figures are reached at weekends during the summer. (These charts are built from spreadsheets containing 17,000 lines and details are sometimes blurred). The lowest recorded electricity use was about 20 GW. The period around Christmas sees reduced demand.

Power use during the day is always higher than at night. In winter, peak demand is in early evening. In summer, demand is flat during the day although is increasingly depressed by solar PV output.  Weekends are always lower than weekdays.

Chart 2. GB national demand compared to estimate maximum wind output in 2035

Chart 2 superimposes the maximum wind output in 2035 and a figure of 90% of this level. The 90% figure is the maximum ever likely to be achieved. The 90% line is, at about 38 GW, greater than maximum demand on all almost all weekend days from April until November.

Chart 3. GB national demand compared to average wind power levels in 2035


The average amount of wind power over the year in the Staffell/Pfenninger analysis will be about 17 GW and this is shown as a red line on Chart 3. The minimum UK demand is over 20 GW, so average supply never matches need.

Chart 4. GB national demand compared to approximate seasonal averages of wind power levels in 2035



The average amount of wind varies through the year. But its variations are approximately the same as electricity demand. In other words, although average wind power is greatest in winter, so is demand (Chart 4). The expected average wind production in each season is a similar proportion of the minimum demand.

Chart 5. GB national demand compared to wind output levels during windiest 5% of the year.

Currently, 5% of the time the capacity factor is at least 68%. The line across Chart 5 shows 68% of the expected 2035 installed wind turbine capacity. On average across the year, the 68% capacity factor will exceed minimum daily demand in all months except the winter.

Chart 6. GB total demand compared to the windiest 5% of the time, adjusted for seasonality in wind speeds

A better way of looking at the relationship between high levels of wind output (the 95th percentile level) and demand is to break the year into the four seasons (Chart 6). Wind variability is greater than seasonal changes in demand. In winter, and autumn particularly, high levels of wind turbine output are more likely to exceed total demand. During almost every day from mid-September to February the 95th percentile wind output is likely to exceed the minimum demand. At weekends and at Christmas, the whole daily demand is sometimes covered by the high wind production.

Very high wind production (at the 95th percentile) would cover 100% of some part of the day’s electricity need over about 200 days a year, mostly in winter and autumn. By contrast, in spring and summer, there will be relatively few days on which wind covers all of the demand at any part of the day because very high winds are much more unusual between April and September.

So what does this mean for the number of days each year on which wind production will exceed today’s need? Very roughly, the analysis in this note shows that about 10-15 nights a year wind will provide all the power that is needed, before even thinking about the remaining nuclear stations, anaerobic digestion, batteries, interconnectors, and hydro. Since Hinkley Point C will probably be paid its full agreed price, even if its electricity is not needed, the additional bills to the electricity consumer should be factored into calculations of the full cost of the proposed new nuclear power station.