UK Weather-Dependent Renewables Reliability and Intermittency in 2021

Gridwatch base data

Gridwatch UK provides selective downloads of UK power generation data at 5 minute intervals.  This posts uses that data directly on a monthly basis.  The chart below shows the profile of that data for April 2021.  This was the moment when the six month UK Wind drought started.

This post uses that 5 minute data to examine the performance of Weather-Dependent Renewables, Wind and Solar power in the UK to date in 2021.  That data is aggregated below showing the monthly percentage contributions to UK power generation.

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The 2021 UK energy generation mix 2021

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Shown below are the percentage shares of power production contribution by generation type on a monthly basis through the last year since October 2020.

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  • Solar:  clearly shows the seasonal variation inevitable with UK solar power resulting in an annual output level overall of ~10% but varying from less than 1% to more than 7%  contribution according to the season.
  • Combined Onshore and Offshore Wind power normally producing ~ 20+% of UK power:  shows the monthly variability with a real Wind drought from April – September 2021, ranging from 9.5% to 15.6% instead of values from 2020 amounting to close to 30% annually.
  • Hydro-Pumped:  on occasions Hydro power can make a contribution of up to 8.5%.  It can be limited by the available rainfall retained in the dams.  It contributed minimal amounts to power supply during the 2021 summer Wind drought, because lower rainfall had not replenished the dam storage.
  • Gas-firing:  At about 41% of the overall power contribution Gas-firing is the mainstay of the UK generation.  Gas-firing is dispatchable to meet demand, and shows considerable variability as Weather-Dependent Renewables are inconsistent in their contribution.  This variability means that it becomes increasingly difficult to maintain profitability of this crucial dispatchable element of UK power.
  • Biomass:  by policy, is considered to not emit CO2 but in fact produces ~3.5 times the emissions of Natural gas for power generation.  This means that the UK use of Biomass particularly at the Drax power complex effectively negates all / any CO2 emissions savings that will have been made using other Weather-Dependent Renewables.  Biomass is dispatchable to meet demand.  In early 2021 it appears to have had several months of limited output.
  • Coal-firing:  in spite of Government policy to cease Coal-fired power generation on the grounds of its CO2 emissions, (about twice that of Natural Gas-firing), some UK Coal-fired power stations have been reopened and have supplied a minor power input of 1.7%, helping to some extent to alleviate the effect of the 2021 Wind drought.
  • Nuclear:  although much of the UK Nuclear generation fleet is being retired progressively this decade, the fleet as a whole still provided a 17+% contribution of CO2 emissions free power in 2021.
  • Net interconnect:  is the summation of all power inputs and outputs to and from the UK.  It clearly shows the crucial dependence that the UK has to imported power from Europe, particularly from Nuclear generated power from France.  From June to August 2021 net imports provided a 10-15% contribution of the UK power supply.  This dependence on imported power poses a real and existential threat to the wellbeing of the UK, especially in the light of current disputes with France.

UK Weather-Dependent Renewables fleet Productivity

The name plate values of UK Weather-Dependent Renewables at the end of 2020 were as follows:

  • Onshore wind ~12.3GW
  • Offshore Wind ~10.0GW
  • Combined Wind power ~22.3GW
  • Solar Power ~12.3GW
  • Combined Weather-Dependent Renewables ~34.6GW

At the full Name Plate value of UK Weather-Dependent Renewables, (Wind and Solar power), amounted to ~34.6Gigawatts in 2021.  This is more than half of the whole installed UK generation fleet and most of this Weather-Dependent generation capacity has been added over the last decade.

The whole 34.6Gigawatts Weather-Dependent Renewables fleet, would provide more than the normal UK demand, were they fully productive.  However, as can be seen above, even in the productive year of 2020 UK Weather-Dependent Renewables only produced ~19.4% of their installed name plate value overall.

2020 was a productive year for Weather-Dependent Renewables in the UK and equally across much of Europe.   The levels of UK Renewables productivity achieved throughout 2020 were as follows:Screenshot 2021-11-04 at 10.16.20.png

Even in the highly productive year 2020 these productivity / capacity percentage values do not compare well with conventional  dispatchable generation technologies, which only loose ~10% of their productivity to routine maintenance.  However, the achieved annual productivity values do not show the detailed problems that the precipitous changes in power output and on occasions more prolonged outages from Weather Dependent Renewables inevitably cause in the management of a stable power grid.

Wind power variations in power output

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The chart above shows a longer history of Weather-Dependent Renewables productivity / capacity % in the UK since since 2002.  Onshore Wind power had a consistent productivity between 20-25%.  Offshore Wind power has been more variable between 41-30% as can be seen above.  Solar power has a regular annual productivity average of <10%.

The chart above shows the Gridwatch data combining, (Onshore and Offshore), Wind power data for the year October 2020 to September 2021.  The productivity / capacity % of Combined Wind power recorded at ~33% in 2020 fell to ~24% in the subsequent year to September 2021, as a result of 2021 Summer Wind drought.   Overall Weather-Dependent Renewables productivity fell from ~25% to ~19%

Not only was there a dearth of Wind production but the chart below shows the acutely precipitous changes the Wind power. These power interruptions are imposed on the power grid by the policy of giving preferential access for Renewables to supply the power grid, (the Renewables Obligation).  The rapidity of the precipitous changes in power output from Wind power can be seen 7-8 April 2021.  This was the start of the six month Wind drought widely affecting UK Wind power production.  As ever more conventional dispatchable generation is lost by Government policies those rapid changes will become ever more dangerous for sustaining Grid stability.

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Thereafter both Onshore and the even the more productive Offshore Wind power were significantly curtailed month by month as can be seen in the 5 minute power output graphs below.

As the Gridwatch data does not distinguish between Onshore and Offshore wind output these figures result from their combined value.  The graphs below show the performance of UK Wind power from January – October 2021.

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  • January:  was relatively productive for the month overall at ~29% Wind productivity / capacity:  for 20% of the month Wind production was less than 10% of overall power produced.
  • February:   was a remarkably productive month overall at ~38% Wind productivity / capacity:  for 19% of the month Wind production was less than 10% of overall power produced with output low late in the month.
  • March:  was a normally productive for the month overall at ~29% Wind productivity / capacity:  for 32% of the month Wind production fell to less than 10% of overall power produced with output remaining low early the month.
  • April:  was a poorly productive for the month overall at ~17.5% Wind productivity / capacity:  for 52% of the month Wind production fell to less than 10% of overall power produced with output falling significantly for the last 2/3 of the month.  April 2021 was the start of major UK and European Wind drought with the Wind Productivity percentage remaining in the range of 19-12% for the whole six month period.  There were extended periods in April 2021 when the combined Onshore and Offshore wind power produced virtually no output.
  • May:  was a poorly productive for the month overall at ~19.7% Wind productivity / capacity %:  for 50% of the time in the month Wind production fell to less than 10% of overall power produced with output curtailed significantly for the last 2/3 of the month.
  • June:  was a poorly productive for the month overall at ~15.7% Wind productivity / capacity:  for 50% of the time in the month Wind production fell to less than 10% of overall power produced with output curtailed  significantly for the last 2/3 of the month.
  • July:  was a very poorly productive for the month overall at ~11.8% Wind productivity / capacity:  for 60% of the month Wind production fell to less than 10% of overall power produced with output substantially curtailed throughout the month.  There were some 11 severe Wind drought days when the combined UK wind power stood virtually idle.
  • August:  Wind productivity recovered to 19% for the month:  for 40% of the month Wind production fell to less than 10% of overall power produced with output substantially curtailed both early and in the second half of the month.
  • September:  was also a poorly productive for the month overall at ~18.5% Wind productivity / capacity:  for ~50% of the month Wind production fell to less than 10% of overall power produced with Wind output substantially curtailed until towards the end of the month.
  • October:  was a productive for the month overall recovering to a full ~34.7% Wind productivity / capacity:  for only ~17% of the month Wind production fell to less than 10% of overall power produced.

Even though UK Wind power is widely distributed North to South and across the British Isles as can be seen above Wind production is far from constant and the output can change precipitously in either direction over very short time scales.  Such variability and the possibility of longish periods, (several days), of virtually nil production makes the management of a consistent power Grid ever more difficult especially when the Renewables obligation insists that Wind or Solar power is used preferentially to supply the Grid, if it is available, even suddenly at short notice.

The graphic below shows the monthly productivity  / capacity % for Weather-Dependent Renewables for the 10 months up to October 2021.  This indicates clearly the poor Wind power performance for the whole six months from April to September 2021 when power output fell to about half of the anticipated power production and in the July it fell to only 1/3 of anticipated normal Wind power.

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Solar power did in part compensate during those low wind summer months, but that support was characteristically intermittent day to night.

UK Solar power generation:  example months January and July

UK installed Solar power amounted to ~12.3 Gigawatts in 2021.  Solar PV power generation shows its expected seasonal performance ranging from 2.6% productivity in January to a summer maximum of ~ 17% productivity.  Solar power viewed on a monthly basis did compensate in part for the six month  2021 Wind drought but it still had the disadvantage of it’s diurnal nature with it’s  substantial fall of in the evenings the times of likely peak load.

Shown below are two example months of Solar power generation, January and July.

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Solar power clearly demonstrates its inevitable diurnal variation: day to night.  In Winter Solar power input with a productivity of 2.6% was be very variable according to Weather conditions, ranging by a factor ~17 times from a poor day to the achieved maximum in the month.  In summer the variation was substantially less only by a factor of ~2.5 times.  Nonetheless throughout the year there is an the inevitable peaking of Solar PV production towards midday, at the times of low demand, and an inevitable rapid fall off towards evening when power demand peaks.


Although the UK 2021 summer Wind drought may have been somewhat unusual, it should be taken as a clear example of what can and will continue to arise with the Weather-Dependency of power supplies.  The illustrations above show:

  • the precipitous variability, the seasonality and the intermittency of Weather-Dependent Renewables

  • any concept of using batteries for power storage has to account for:
    • sudden and rapid variations of power input both from Wind Power and Solar inputs
    • accommodation of long-term reductions of Wind power extending for several days
    • the large long-term seasonal variations associated with Solar power
    • there has to be excess power available to maintain any batteries at full charge ready for use:  that is unlikely to happen in periods of Wind drought.
    • likely costs of batteries of sufficient size to account to Weather-Dependent Renewable variabilities:  it is estimated that a battery installation of sufficient capacity to cover a 10 day Wind drought as shown above would require about 5 Terawatt hours costing about £1,500 billion, ie, ~1.5 times annual UK public spend.

On the storage of renewable electricity

The provision of a stable National power supply should not have to contend with such an unreliable resource to provide its crucial National service.

The Appalling Delusion

The late Professor Sir David Mackay, (former chef scientific advisor of the UK Department of Energy and Climate Change), in a final interview before his untimely death in 2016 said that the concept of powering a developed country with Weather-Dependent Renewable energy was:

“an appalling delusion”

Minute 13 onwards.

Weather-Dependent Renewable Energy depends on capturing dilute and very variable sources of power.  Weather-Dependent Renewables are thus both capital, land use and maintenance expensive and inevitably unreliable.  Weather-Dependent Renewables are universally more expensive than the conventional alternatives of Nuclear power or Gas-firing.

At the time Professor David Mackay also said:

“there’s so much delusion, it’s so dangerous for humanity that people allow themselves to have such delusions, that they are willing to not think carefully about the numbers, and the reality of the laws of physics and the reality of engineering….humanity does need to pay attention to arithmetic and the laws of physics.”

and later in the same interview he said:

“if it is possible to get through the winter in the UK with low CO2 Nuclear and possibly with Carbon Capture and Storage, there is no point in having any Wind or Solar power at all in the UK generation mix.  This is especially so for Solar energy:  the UK is one of the darkest nations on earth and produces about 1/7th of the power output in winter as in the summer.”