Findings from a typical domestic PV generation system – one year in

I have now collected a year’s worth of data on how my modest, 1.68kW peak PV system has performed over the last twelve months, and it makes for interesting reading.

Over the period October 2011 through to the end of September 2012, I generated a total of 1075 kWhs.  Across that same period I consumed a total of 2218 kWhs of electricity (a large jump upwards from years gone by and I’m at a bit of a loss to explain where that increase has occurred – but that’s for another blog post).  So in total I have generated a little less than half my yearly demand.

My new backstop meter, installed at the end of September 2011, informs me that my total net electricity demand over the last 12 months, that is from the grid, totaled 1430 kWhs.

Hence that in turn that tells me that my PV system has fulfilled approx 35% of my total electricity use over that time period; not bad for a small system!

Even more encouragingly, it also shows that I have managed to locally utilise around 75% of my total PV generating capacity.  Only 26.5% was exported out to the grid.  We have been changing our habits and behaviours to maximise usage and also, as I work from home for a significant proportion of the day, I am able to take advantage of sunny afternoons to put the washing on, and to cook meals, but regardless I’m particularly proud of this statistic.

When we come to payback times, the data isn’t so encouraging.  An approximate 1000 kWh annual output has garnered me around £450 in FiT payments this year, including the extra 3pence for 50% of the generating capacity that is assumed is put back into the grid (for me that is generous as I’m only putting back 25%!).

The 788 kWh of free PV electricity I have used has saved me around £100 in annual electricity charges.

Put together that adds up to a total of £550 savings per annum.  On a £8000 original outlay, it will take around 15 years to pay back the original capital cost.  Far greater than the sub 10 year claims that abound in the industry.  This doesn’t overly concern me, as there is another 10 years of useful service out of the system after all original costs are paid off, and the point of having the system wasn’t just to get a quick payback.  But it is an interesting finding, and I would very interested in hearing from other PV owners to see if I am typical or whether my smaller than average system, and the fact I do not have a perfectly due south aspect has reduced my generating capacity to a significant extent?

So, please get in touch if you have an interesting PV generation story to tell.  is your system over- or under- performing? and if so, why do you think that is?

Other related blogposts on PV generation and ownership by paula owen consulting:

Being a PV generator: Lessons I have learnt Part 1

Being a PV generator: Lesson learnt Part 2

PV systems and meters running backwards: need for clear guidance


10 comments… add one
  • Duncan Booth October 5, 2012, 10:44 am

    A data point for comparison: I have 2.8kWp solar panels installed at the end of January this year, so they've had a bit over 8 months. So far I've generated 2047kWh which is worth £908 in FiT and export tariffs. The installation cost just under £8700 so payback should be under 9 years (depending on how much more I get in the remainder of the year and how much inflation increases the FiT).
    I don't know how much I've exported, but most days there's nobody in during the day so I'm pretty sure I'm sure I'm subsidising you for the export.

    • Paula October 5, 2012, 10:54 am

      Thanks Duncan

      Interesting to compare.  My system was pretty expensive, c.f. yours, as it was installed before the prices started to tumble when the uncertainty about FiT came in, and they were high spec at the time 240 watts/panel, as i had such little available space to place the panels. So you got an extra kilowatt for the same sort of price which is great.

      Re the export, you can measure it if you are prepared to pay for a Solar Wattson, as this gives you your total daily usage, not just net grid usage, but they are pretty expensive!  I’m not sure if cheaper solar-enabled real time display units will do that?  You may well be subsidising my lower than average export back into the grid – but my 25% extra home usage, on the 3pence a kWh exported, is only worth £7.50 per year so hopefully you don’t feel too aggrieved 😉

      Good luck with your system

  • Dave October 8, 2012, 10:44 am

    I would be wary of relying upon any current-clamp meter (such as WattsOn) for measuring household consumption and/or export. Current clamps are quite inaccurate, particularly at low currents (ie low power usage). We only use our OWL meter as an indicator of our import/export now we've got PV (there's no single point we can clamp to measure household usage).
    This may account for your higher consumption and hence may be underestimating your export %. Most households who have PV installed see a reduction in electricity consumption (as well as import) due to improved awareness, eg switching the lights off.
    We're just under 2 weeks away from our commissioning anniversary of our 3.68kWp SSE system on a 45deg roof with no shading in Somerset. Generation to date has been 3430kWh, which is 16% above SAP annual estimate and 7% above our PVGIS-Classic estimate and 5% above PVGIS-Climate-SAF estimate. Unless the next two weeks are rubbish, we should achieve about 9% above PVGIS-Classic.
    We don't have a full year of electricity consumption data prior to our install date as we only moved in ~10months before. Due to the lack of export meter or reliable measurement of export, we can also only guess the amount of export if we assume our consumption hasn't changed (although it is probably lower, meaning export is higher). Our annual import of electricity (extrapolated from the 'short' year prior to install and the almost full year with PV) is down from 3900kWh to 2860kWh (down 27%).
    Based on the above assumption, we've used around 1000kWh of our generation or 29%, so have exported 71% (or quite likely more if our consumption has reduced due to better awareness).
    We had an ImmerSUN installed in September to make greater use of our excess generation, which diverts what would have been exported to the grid into the immersion in our 210litre hot water tank (until it is hot). So far results have been encouraging, with the ImmerSUN diverting 39% (74kWh) of our generation since then into heating our water instead of exporting to the grid. Every export kWh diverted is at least that amount of kWh saved in gas usage (or more likely more due to boiler inefficiencies), but it's too early to quantify the savings just yet.
    Our return to date is 17% against the total outlay, which included the PV system, Building Control inspection, Planning Permission check (to confirm in wiriting it was permitted development), moving the TV aerial from inside the loft to a chimney, a GEO Solo PV monitor and the ImmerSUN. That return is based on the FiT income alone, and ignores the import savings on electricity and now gas.

  • Richard October 26, 2012, 1:39 pm

    Hi Paula
    I hope this finds you well.
    Our 2.82kWp Sanyo hybrid system was commissioned at the beginning of February this year. When I sent off for our 2nd FiT payment on 13 September we had generated a total of 1995kWh (£928 received). Our roof is SSW facing (bearing 220deg) with a 40deg pitch, so not absolutely ideal but pretty good. Our capital outlay was £10972 (Duncan's deal was an excellent one) and I'm anticipating simple payback year 9-10. That's pretty much in line with the results from my model. Incidentaly, I also did a DCF calculation at a 4% hurdle rate which suggested discounted payback in year 12 – including a replacement inverter in that time-frame..
    Net demand over the above period was 1475kWh. I don't have a measure of total demand but looking back at a similar period last year I estimate that, ceteris paribus, it should be about 2100kWh so we've exported something like 70% of generation. That surprises me a bit 'cos we've been pretty good about running the washine machine, dishwasher etc. during the daytime. Cooking we haven't time-shifted and that still takes place early evening. I haven't actually sat down to calculate what I'd expect the demand from dishwashing etc. to be. With just 2 of us in the  house I'd expect our usage to be similar to yours so it's not unreasonable that my larger system should spill more to the grid.
    Incidentally, I've been monitoring gas and electricity usage since before we had our extension done. We increased our floorspace by about 50%, improved the insulation (there's still a bit of bare solid wall), swapped from a D- to an A-rated boiler and installed 3 solar hot water panels. Our annual gas consumption has hardly changed which one might argue is just a balance between having to heat a larger space and doing it more efficiently. The detail is a bit more interesting. Our peak demand in winter is higher (the space heating effect, I guess) whereas in summer we see a slightly deeper trough but also a longer lasting one (solar panels coming into their own, I think).
    The electricity story is a better one – our consumption (pre-PV) is down by about 12%. That will be down to moving almost totally to cfls (I will admit to a few wicked halogen downlighters) and to not having to resort to top-up electrical heating in our lounge on the worst winter nights. Don't ask me to estimate payback on any of this – the project was a mega-one and disentangling the finances would be a nightmare.

    • Paula October 26, 2012, 3:17 pm

      hi Richard

      Fascinating, thank you!!  It’s good to know I’m not alone in energy usage geekness!  Interesting you are exporting so much to grid, but I guess it isn’t surprising with larger systems – you simply cannot use it all when it’s at peak generation as its’ in the day when people are generally out.  We are all crying out for a cheap, robust small battery solution i guess.

      I’m finding that my generation has fallen off a cliff this October, lucky if i’m generating one kWh per day!

      All the best Paula

  • Richard October 27, 2012, 7:44 am

    Hi Paula Yup – dull days are bad news! Re storage, have you seen this ? R

  • Bruce December 10, 2012, 1:08 pm

    Hi Paula, forgive me for asking (if I've missed the details of the installation elsewhere here) but where in the country are you and whats the layout/orientation/shading on your array?  The figures seem quite low-if you have shading has it been mitigated against by the use of multiple MPPTs (probably not on a system of your size) or voltage optimisers?  Can do a few calcs for you?
    Best wishes

  • Bruce December 10, 2012, 1:11 pm

    Hmm, frowns…the figures seem a bit low…would you like me to do some calcs?

  • Tom Slaiter February 8, 2013, 9:40 am

    I agree with Duncan, though I love the post thanks so much for the info and help 🙂

    Great post

  • Jerry August 7, 2013, 7:15 pm

    We have a 3.91kW system installed on a due south facing roof with no shading. In the last year before installation we used 3743kWh and in the year just ended, the 1st measurable since installation ( trouble with a non backstop meter which they were slow to replace), we used 2155kWh a 40% reduction. In that period we also generated 3649kWh, which exceeds the estimate made before installation. We are in all day so were able to save more than the average family by switching as much electricity use to daytime as possible. Yes, I know, I've become fanatical!


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