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Solar power in the UK

When thinking of backup electrical power for during/after an event solar has a number of things that make it initially seem attractive:

  • No fuel (well sunlight but we’ll get to that)
  • No noise (good OPSEC)
  • Low visibility (good OPSEC)
  • Long life expectancy (many decades)
  • Miniscule maintenance (an occasional wipe down if it gets dirty)

However the story isn’t all good.

Some statistics I found from trawling the web:

  • Average hours of daylight (each day) in Britain by month
  • January 8
    February 9
    March 11
    April 13
    May 15
    June 16
    July 16.5
    August 16
    September 14
    October 11
    November 10
    December 8

  • Average hours of sunshine per year in the UK is 1350 (3.7hours/day on average)
  • Solar in the south of England typically achieves about 3% of maximum rated.

Yes that last one said 3% that’s not a typo and that’s in the south of England (things get even worse the further north you go).
Daylight produces very small amounts of power compared to sunlight.

The first thing we need to accept is that Photo Voltaic Solar cells are rated for an unattainable ideal. They are rated at 25°C when experiencing the sort of sun they would get on the equator at mid-day on a cloudless clear day.

If you’re using a PV module to charge batteries (rather than using the power directly) then once the Voltage out of the cell drops below the battery Voltage you get no power into the battery, so marginal power means no power.

I’ve been doing some analysis of a PV cell since the start of the year and the results are pretty disappointing.
For a “100W” solar cell charging a 90Ah 12V ‘car’ battery. Power out of the solar cell per month is as follows (units of Watt hours per day):

January 18 Wh/day
February 27 Wh/day
March 80 Wh/day
April 150 Wh/day
May 183 Wh/day
June 180 Wh/day
July 225 Wh/day
August 149 Wh/day
September 113 Wh/day
October 46 Wh/day
November 17 Wh/day
December 11 Wh/day

I expect the back half of the year to dip back down towards Januarys levels pretty symmetrically with the first half.

If the solar cell was meeting ‘ideal’ conditions 100W for 8 hours would produce 800Wh per day in January. I achieved a little over 2% of that ‘theoretical ideal’. Pretty close to the 3% mentioned above.

Before we rush off to use the 18Wh in January we need to consider losses in the system. Not all the power out of the solar cell made it into the battery, there is a charge controller and it has losses.
The Solar charger has losses of about 2.3Wh/day
The battery also experiences ‘self discharge’, the amount of power that is lost in the battery when it’s doing nothing. That’s difficult to measure but can be estimated as 40%/year or 0.15%/day (for a new battery, it gets worse as the battery ages) which on a 90Ah battery is equivalent to about 2.9Wh/day.
The battery also converts some of the energy it receives into heat, it’s only about 70% efficient at storing the energy it receives.

So the 18Wh per day (average) for January leaves is with ((18*-2.3)*0.7)-2.9) = 8Wh to use.

Lets see what we could do with 8Wh. Once a day I could do one of these:

  • Power a radio Radio (receiving) for about 8 hours.
  • Run a single cluster LED light (1W) for 8 hours.
  • Run a single ‘normal’ 12V MR16 light (30W) for 16 minutes.
  • Run my Fridge (92W at 230V say 130W through an inverter) for a little under 3 minutes (it needs to run about 8 hours a day)
  • Bring about 80mL of water to the boil.
  • Run a typical ‘fan heater’ for about 15 seconds.
  • Charge about 4 * AAA or 2 * AA NiMh batteries (assuming a very efficient charger, most aren’t).

Clearly 100W panel isn’t going to cut it for anything except listening for radio broadcasts and a little light in the dark evenings and even then we need efficient (and dim) lamps!

Of course during the summer we could have 15 to 20 times the energy that we get in January but we need to design for the worst case not the best or even the average.

Since a 100W Solar panel presently costs about £350 very few of us can afford to go to huge solar arrays.

To give my Fridge the 800Wh a day it typically needs I’d need at least 7000W of Solar panel (in Jan) with a cost of about £16 000 (economies of scale mean this costs less than the £24.5k for buying 70*100W units) I’d also need a lot more batteries and 49 m^2 of space to mount the panels (Assuming about 0.7m^2/100W)

When looking at solar power realistically we need to consider it for lighting and communications.
For cooking and heating we’re going to need to burn things, probably wood.
For preserving food we need an alternative to refrigeration. For long term grid down scenario our fridges are going to break anyway and they’re not easy to repair or re-build.

9 comments to Solar power in the UK

  • grumpy old man

    hi skvez
    wow! guess i what i won’t be using then. LOL
    in my plans power will not be needed i have a genny but i know it will not last for long, did think solar and know so one who lives off the grid for most things in a caravan, but guessed knowing my luck it’s best to plan for no power basic ray the way for me.
    now if this brill site shows me how to do it, then that’s a different story!
    althought i have been working on a power inverter, 12v battery an electric motor and alt. from a genny and alt from a car if i get it sort it i will post plans fo you all to try.

  • Skean Dhude


    I still think solar is one way to go. We minimise our requirements and use solar where we can because of the benefits. Wind is a load of hot air 🙂 but even that may be useful to you in a survival situation.

    It is about finding what is best for your unique situation. There is always a way.

    Look forward to seeing your plans.

  • iaaems

    There is a web site which deals with alternative or free energy. As far as I can tell it is based in the UK and it is a huge database of information from all over the world. At the very least it is thought provoking. It is run by a gentleman by the name of Patrick Kelly and can be found at
    I have no commercial interest here – just passing on information that I feel might help in some way.

  • Skvez


    Solar has its place and it’s excellent for what it does but the point of the article is to clarify what it can and can’t reasonably be expected to do.
    Too many people design systems based on the solar cell rating and overlook the realistic value and the losses in the system.
    They are going to be in for a nasty shock if they’re relying on such a system for something critical such as keeping stocks of medicine cool.


    I’ve an article on wind planned, I’ll not say “coming soon” but hopefully coming sometime. Again wind has its place provided your expectations of it are realistic.


  • Skean Dhude


    Thanks for that. These things are always interesting.


    Excellent. Your articles are always interesting.

    I have low expectations of wind for home use but consider it a viable option for survival.

  • Skvez

    I wanted to complete the table for the Solar performance in 2011
    Aug 149 Wh/day
    Sep 113 Wh/day
    Oct 46 Wh/day
    Nov 17 Wh/day
    Dec 11 Wh/day

  • Skean Dhude


    Thanks for that.

  • Paul

    Solar is OK but what you going to do with it?
    Most small panels generate 12 volts (unless you’ve gone mad and brought these whole roof generating stations).
    So you’ll need to charge a battery. From there you use an invertor to generate mains voltages. Invertors are poor performers. They lose efficency through heat loss and in ‘cheapie’ ones typically 35% of the current in gets radiated away as heat.

    Take a perfect world. You need 690 watts to run your central heating pump (Second speed setting on my pump). Thats 3 amps at 230 volts. or 57.5 amps from a 12 volt car battery. Ain’t going to last long is it? Now add in the average invertor losses of 35% and you now need 77 amps out of the battery to get the same power. Using my car battery (140AMH), the pump runs for 2 hours (just).
    Me, my personal survival is built round a little 750w petrol generator. Lot less trouble and it also recharges batteries.
    As for fuel, its a very old generator so also runs on wood alcohol. “Interesting” that was too but that’s another story.

  • Skean Dhude


    And I can’t wait to read it as I’m looking at doing that.

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