The impact of clouds on your solar PV system

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We have been running an experiment with one of our partners, Element Engineering, to log the power output (in Watts) from a domestic scale solar PV system.  The system is pretty typical in terms of its type and scale - mono-crystalline panels (rather than poly-crystalline so better in shady conditions), 2.45 kW Peak system and located in mid-Scotland.  But the difference between this and the many other graphs you'll have seen is that we have logged output data at 2 minute intervals over a period of 4 days to show how output varies over small periods.  We haven’t seen this done within the solar PV industry - there may be reasons why this is the case.  We wanted to explore the issue of output intermittency - how much output varies over short periods, litterally minute-by-minute.  We also suspect similar intermittency over even shorter (2-5 seconds) periods too.

Solar PV output graph at 2 minute intervals

Our conclusions from this experiment are relatively profound:

  • A Solar PV system that has been sold to you with a ‘3 kW Peak’ capacity can, if well sited and oriented, generate 3 kW or more.  However this is only likely on rare occassions when there is 'optimal' sunshine conditions when the panels are relatively new (as performance year-on-year becomes marginally degraded).  The ability of a solar Pv solution to generate power will be dependent on a number of factors including amount of solar radiation hitting the panels at a point in time (impacted by weather, angle of the sun etc.), the temperature of the panels, the efficiency of the wiring, efficiency of the inverter, the age of the panels, snow/dirt/bird droppings on panels etc.
  • Solar PV does generate some electricity in daylight hours BUT output is highly variable depending on the type and percentage of cloud cover - optimal output will only be achieved on days with continuous full sun and no cloud cover.
  • Outputs go up and down minute-by-minute (and every few seconds) by up to 75% as a result of cloud cover – this includes thin cloud or intermittent cloud cover (caused by small clouds) on a largely sunny day.
  • Outputs can vary hugely within the period of daylight during a single day depending on weather conditions (unless they are completely cloud free).

The implications of this in terms of optimising your investment in solar PV from maximising your local consumption are:

  • The concept of “free electricity” may be a compelling concept on advertisements but, of course, the reality is nothing like as appealing.  It’s all down to our ability to actually use what the solar PV system generates - in real time.  Chances are you are only likely to use about 20-25% of it thereby cutting your electricity bills by a similar proportion.  Much of the reason is down to UK weather patterns (and seasonal variation) and our patterns of energy demand – we are far more ‘needy’ of electricity during our colder seasons and during hours of semi- or full-darkness.  Most of us use much less electricity during the day in sunny weather than we do in winter or at night – we turn on less lights and we might dry clothes on the line rather than in a tumble drier, for example.  When it’s sunny, it is realistic to expect that a baseload of computers, lights, fridges, freezers etc will be covered by a solar PV system most of the time. [In fact your exported electricity is actually offering an environmental benefit to all your neighbours who are able to use the ‘green energy’ generated by you to support their ‘baseload’ demand.]
  • We would suggest that if you are anywhere close to consuming 25-30% of your solar PV generated energy you are doing well especially if your system is closer to 4 kW or larger.
  • If you are thinking about installing a solar PV solution then to more accurately calculate the financial returns using an online calculator then you should use (like Solar Guide) that allows you to edit their default assumptions in terms of the "% of energy generated that will be used in my home".  A more conservative estimate would be 20%. 
  • If your focus is lowering your energy bills and you have a fairly standard sub-4 kW system, the idea of switching away from gas powered devices to use electrical devices (eg. for space or water heating) that require to be left on for significant periods could well be a false benefit.  Unless you have a large number of cloud-free days (unlikely in the UK with the possible exception of south-west England), these devices are likely to be balancing their demands by pulling electricity from the grid during periods of cloud cover.  It would be advisable not to swap your gas cooker for an electric one because you’ve just installed a solar PV system.  If you have a gas boiler, it may also not be sensible to use an electric immersion heater on a timer to heat your water during the day – it is likely to be much cheaper using gas.
  • If you are on a set tariff price for all the electricity you use, it does make sense to switch on energy hungry appliances like tumble driers and washing machines during the daytime (in sequence rather than all at the same time).  However they may not be consuming as much of your ‘home-grown’ energy as you may think.  If it’s not a cloud-free sky then your PV outputs are going up and down minute-by-minute.  So, while you will certainly be reducing the amount of grid-sourced electricity they consume, they are still going to be dependent on electricity from the grid.
  • The variability of output from solar PV is always going to make it difficult for households to consume more than 30% of what they generate.  It will be even more difficult to get close to consuming 50% or more of what you generate.

There are some wider implications:

(1) If you are a household considering a "free energy" deal through a ‘rent-a-roof’ scheme then you may be able to cut the amount of electricity you have to buy from the grid by around 20-30% (although this may not translate directly into 20-30% savings on your bill).   You will also be helping your neighbours to ‘green’ their electricity supply. 

(2) Any efforts you switch to appliances and devices that use electricity instead of oil or gas alternatives may simply result in importing even more electricity from the grid than you would normally do leading to bigger (rather than smaller) energy bills.

(3) The ‘deeming’ arrangement does appear to be slanted in favour of energy companies over micro-generators although the amounts of money are small enough to be over-looked by many people.  Energy companies may also feel they incur additional overheads to process Feed-in Tariff registrations which they need to re-coup.  Micro-generators should really be offered the ability to ‘opt out’ of deeming and be paid an export rate nearer 5p/unit (roughly the wholesale price) without any additional costs for installing and maintaining export meters.  Perhaps an easier change would be to revise the ‘deeming’ arrangement to be closer to 70:30 (70% to grid and 30% to household) rather than 50:50 – this would mean generators would be paid 50% more than they are currently getting for the electricity they export.  For domestic scale solutions we are talking small amounts here.  If you generate 1000 kWh from a typical domestic scale PV system, you export 80% (800 kWh), this should mean to would get £25 annually (3p x 800) in grid export payments.  But in reality you only get about £15 (3p x 500).  So many households will be losing out by around £10 annually.  If it takes until 2020 to get your smart meter then it does start adding up. The larger your system the larger this loss is likely to be while you still 'opt in'.

The introduction of smart meters and smarter backend processes by the energy companies should make ‘deeming’ a thing of the past but it may be several years before you, the micro-generator, get the opportunity to have a smart meter installed.  

Read more about the pros and cons of ‘opting in’ and ‘opting out’ of the deeming process.

Add your tips about optimising your solar PV system to cut your energy bills on our discussion pages.

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Maximising the potential of public assets

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On the 21st of June The Scottish Council for Development and Industry (SCDI), in partnership with COSLA and SLAED, organised a conference including industry, publicly sector bodies, and trade bodies with an international twist given by the ex Mayor of Naestved, Denmark.

The aim of the conference was to explore ’Maximising the potential of Public Assets for a Low Carbon Economy (LCE)’. The SCDI recognised the untapped potential of Local Authorities to implement and profit from the LCE through the government incentives of FIT and RHI.  The core aims of the conference was to share knowledge and experiences of organisations who have researched and/or implemented Low Carbon Solutions.

GEN's Nick Yeats reports back from the conference finding a number of rich seams of information for Local Authorities. See our GEN SCDI Conference Report.