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The Effect Of Clouds On Solar Power

January 15, 2022

By Paul Homewood


There’s a good article on TCW about the giant new solar farm proposed in Cambridgeshire:




We are naturally all familiar with the intermittency of solar power in terms of seasonal and day/night. But Julian Flood also raises the issue of reduced generation during cloudy or foggy weather.

I have heard this idea pooh poohed by renewable enthusiasts, who argue that it makes little difference – something about photons apparently!

However a look at this weeks solar power data shows this is nonsense.


The last three days have been sunny, but Monday and Tuesday were rather dull, though certainly not foggy here.

As a result solar power, which peaked at over 4 GW Wednesday to Friday, did not even reach 1 GW on Monday.

  1. January 15, 2022 4:53 pm

    The ruinables industry is adept at telling lies. It will say anything to get wind and solar farms built and fleece the electricity consumer. Unfortunately our green government accepts all the greenwash put out by the ruinables industry.

    Did anybody ever vote for all these ruinables which are devastating the countryside and destroying farmland, therefore causing both fuel and food poverty? Ruinables developers are beneath contempt.

  2. MrGrimNasty permalink
    January 15, 2022 5:07 pm

    I would expect the drop is less pronounced when the sun is at a higher angle.

    This time of year solar is near pointless regardless.

    Wind has been pretty useless for nearly 2 days again too.

    • Ray Sanders permalink
      January 15, 2022 7:36 pm

      “I would expect the drop is less pronounced when the sun is at a higher angle” I am curious to know why you would “expect” that. The PVlive website quoted above shows huge daily variation even in spring/summer. May 8th 2021 was just 12.6GWh whilst April 23rd was 79GWh- a massive variation. Switch the parameters to daily aggregated and time scale to one year and the daily variations are quite remarkable.

      • MrGrimNasty permalink
        January 15, 2022 8:56 pm

        You’ll note in seasons when the sun has a low angle of incidence the range of max to min generation is nearly 100%, whereas at times when the sun has a high angle of incidence it’s never near 100% loss.

    • Julian Flood permalink
      January 16, 2022 10:12 am

      Solar works best in summer when it is needed least, unless you live in a green house which harvests the sun’s rays and is insulated to within an inch of its life — then you can use your heat pumps ion reverse to keep cool.

      I think I’m joking but I’m not sure.


      • Gerry, England permalink
        January 16, 2022 10:56 am

        And during the day when demand is also at a low – hence the famous Duck Curve. California has a massive over-supply of solar electricity such that is has to even pay neighbouring states to take it.

  3. January 15, 2022 5:24 pm

    Fog is probably a special case but cloudy days give more solar power than you might think, and more constant through the day, due to reflection. A panel pointing away from the sun can easily give MORE *instantaneous* output on a cloudy day than on a cloudless day, but the overall energy produced on a cloudy day is very roughly around 10% of what you get on a cloudless day, for a properly aligned panel.

    • Harry Passfield permalink
      January 16, 2022 11:57 am

      But a ‘properly-aligned panel’ is like a stopped watch. Unless the panel is on a gimbal that will allow for sun-tracking the panel will only be 100 % right at one point in the solar day.

    • Nial permalink
      January 17, 2022 9:30 am

      “A panel pointing away from the sun can easily give MORE *instantaneous* output on a cloudy day than on a cloudless day”

      If it does that the output must be laughably low on a cloudless day?

  4. 2hmp permalink
    January 15, 2022 5:42 pm

    Lindzen and Soon have done all the research on this. Clouds and vapour trails cool the atmosphere as well as reduce the albedo.

  5. Malcolm permalink
    January 15, 2022 7:04 pm

    Paul, have a look at and see how little wind actually contributes to the daily energy demand on the National Grid.

  6. January 15, 2022 7:18 pm

    Do people want their power supplies to be a permanent lottery or not? Time to choose a final answer.

    • Phoenix44 permalink
      January 16, 2022 8:49 am

      Strictly speaking a lottery is one in which the winner is picked from the tickets sold. The National Lottery is oddly not a lottery! Energy supply is more like Euromillions – I pay to have a chance of winning but there’s no guarantee anybody will win, let alone me.

  7. Dave Ward permalink
    January 15, 2022 7:34 pm

    You’ll often see companies claims that “Our panels continue to work on cloudy days” but they NEVER back that up with details – I know why:

    I used a small 15 watt panel at a remote building for many years. Its job was to keep a 12 volt battery charged, which in turn powered an ultrasonic pest scarer. I carefully aimed it for best results both during the summer and winter months, and I also fitted a digital meter to see what current it was producing. The absolute best on a sunny July afternoon was 1 amp. Even then, a cumulus cloud crossing the sun would knock it back to around 6-700mA. Overcast summer days would give 3-500mA, and a gloomy winter day might provide 100mA – if that. Suffice to say, during winter months, I would have to take the battery home to properly recharge it every couple of weeks.

    • calnorth permalink
      January 16, 2022 10:07 am

      I experience the same with a panel on a garden shed. 5 to 6 months of poor results. Am in W. Mids.

  8. John Peter permalink
    January 15, 2022 8:22 pm

    “The ruinables industry is adept at telling lies. It will say anything to get wind and solar farms built and fleece the electricity consumer.”

    I wonder if a successor non zero carbon government can use some of that to claim contract non conformance and reduce or cancel contracts leaving the owners to face repayments or compensation for ‘misrepresentation of facts’. Problem is: we don’t have a party not signed up to the climate emergency. We need Sunak to take over and to call his friends in India and ask them why they ignore COP26. They may tell him they are following the science such as this

    “135+ Papers Find Extremely Low CO2 Climate Sensitivity”

    • Jack Broughton permalink
      January 15, 2022 8:49 pm

      I’ve generally followed this topic for several years, but did not realise that there were so many publications querying the basis of all the computer models that predict imminent doom by using fake forcing functions.

      On a quick scan, I did not see a reference to David Coe’s excellent work using HITRANS, which is the first paper based on basic transmission physics so far as I know.

      It is amusing that Hoyt C. Hottel showed that CO2 radiation was reduced when moisture was present back in the 1940s.

  9. Devoncamel permalink
    January 15, 2022 9:05 pm

    Most contributors here believe the claimed AGW argument to be grossly misleading at best, if not downright false. Unreliables, as we know, attract massive subsidy. Our green washed politicians and civil service have created a feeding frenzy for opportunists, even relaxing planning and safety regulations for good measure. Flood is right, the whole thing is an ideological scam.

  10. David Wojick permalink
    January 15, 2022 9:21 pm

    The rule of thumb is that solar runs at 10% to 25% on cloudy days, depending on how dense the clouds are. On partly cloudy days it oscillates.

    • David Wojick permalink
      January 15, 2022 10:03 pm

      In your graph the smallest spike is about 10% of your biggest. This could all be due to cloudiness.

    • Sam Duncan permalink
      January 16, 2022 1:46 pm

      Sounds about right. I first realised the extent to which clouds block light during the solar eclipse of 1999. While it was total across most of England, here at 54°N it was about 80%. And it looked much like a cloudy day. I imagine that it’s possible for solar panels to pick up non-visible wavelengths that perhaps aren’t affected so much by cloud, but I’ve been sceptical of them ever since.

  11. January 15, 2022 10:14 pm

    Land may be seized to make way for solar farms in net zero drive

    Homeowners and farmers are being threatened with having their land effectively confiscated to make way for solar farms to meet Britain’s net zero target, The Telegraph can disclose.

    Energy firm Sunnica has submitted plans to build a 2,792 acre solar farm and energy storage infrastructure on the Suffolk and Cambridgeshire borders.
    – – –
    Time these money-grabbing bandits were reined in.

    • Ray Sanders permalink
      January 15, 2022 10:50 pm

      This map allows you to zoom in to all UK transmission and distribution lines from 400kV right down to 6kV. It is worth noting that solar farms are homing in on existing grid connection points.
      The Cleve Hill huge solar farm and battery near Faversham in Kent is not there due to the land or solar quality but because the London Array has its grid connection point there which makes it an almost free connection.
      This map is a good clue as to where all future grid or distribution network solar (and wind turbines) are going to be located .

  12. dodgy geezer permalink
    January 16, 2022 7:18 am

    None of this is a problem for the authorities.

    Smart metering will force demand to adapt to supply…

    • January 16, 2022 9:03 am

      Mr Geezer,

      smart metering is rationing. It also will not work. It is another uncontrolled and disruptive aggravation for the grid.. What happens when all the available load is switched of but supply is still lacking, which will happen?

  13. Ben Vorlich permalink
    January 16, 2022 8:01 am

    The last three days have not been sunny in the bit of the Trent Valley where I live. We’ve had 3 days of freezing fog finally gone this morning

    Then I read this

    . The Met Office warns of armed militias roaming a UK ravaged by climate change in doomsday report (but maybe they should get this week’s weather right first!)

    • Phoenix44 permalink
      January 16, 2022 8:52 am

      When did climate scientists become experts at forecasting economies and political events?
      Indeed when did anybody become expert at that?

      • January 16, 2022 4:50 pm

        J K Galbraith – “There are only 2 types of forecasters; those who are wrong and those who don’t know they are wrong”

    • John Peter permalink
      January 16, 2022 9:05 am

      At least that would be a good script for a Hollywood film.

    • January 16, 2022 9:41 am

      A country-wide “system black” is what everyone should be worrying about, it would make the London riots (aka looting spree) look like a tea party.

    • Julian Flood permalink
      January 16, 2022 10:07 am

      I think they mean that after a big power crash (crisis, catastrophe, emergency, Armageddon (choose any apocalyptic word from the Climate Crisis Dictionary, available from our sponsor) things will turn unpleasant, so we’ll have to pretend it’s not us and our suicidal policies…


    • Mack permalink
      January 16, 2022 10:23 am

      Wow. How original that 6th form dropouts from the Workers Revolutionary Party, and Extinction Rebellion currently masquerading as Met Office wonks and Cambridge University researchers, call for the establishment of a 21st century Committe of Public Safety to protect us from climate change. And a so called ‘conservative’ government funds this crap? Unbelievable.

    • Gerry, England permalink
      January 16, 2022 11:01 am

      There is truth in the report but NOT for the reasons they give. Continuing with insane efforts to stop global warming will result in the apocalypse they describe as the economy is destroyed.

  14. Coeur de Lion permalink
    January 16, 2022 9:53 am

    Solar panels work best when demand matches supply. Viz Saudi and Californian air conditioning. Elsewhere waste of rations

    • It doesn't add up... permalink
      January 16, 2022 1:11 pm

      You might expect to include somewhere like South Australia too, for similar reasons. Yet the average value of rooftop solar is close to zero, and utility solar is only positive because they curtail during periods of negative prices.

  15. Nial permalink
    January 17, 2022 10:05 am

    There used to be a website “Variable Pitch” that listed the historical outputs from many (mostly) renewable electricty sources throughout the country. I’ve just found this on the wayback machine….

    This showed the output of our local ‘Solar meadow’ at Edinburgh College was almost 0 over three winter months. Unfortunately the source figures haven’t been archived.

    A couple of years ago Variablepitch was taken down, after a threat of legal action I believe.

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