Solar Panel Output Degrades/Reduces over time - How Much?

jimjames

The standard info provided with solar panels shows that they can lose output over time, I believe around 0.5% per year. My panels are supposedly warranted to still produce 90% output after 12 years. Interestingly the annual variation in sunshine days across the year means that although months can be very erratic the overall annual output is fairly stable and there is no obvious decrease in the output that could be attributed to the panels losing efficiency. For our 3kWp panels the total has varied from around 3000kWh to 3350 kWh over the 9 years we've had them as per table below. 
http://solar-panels-review.321web.co.uk/index.php/yearly-comparison-of-solar-pv
Has anyone noticed the output decreasing from their panels, maybe someone who has had them longer than 10 years? I guess that's a fairly small number of people when the FIT scheme only started around 10 years ago.

EricMears

Can't manage 10+ I'm afraid but we have had them since 2011.
I was slightly worried to see that last two months have been below 80% of the pvgis suggestion but April & May were both >120% of it so relaxing somewhat.  Looking back,  I see that November is practically always below pvgis and this years 20% down is a lot better than last years 35% down.  Even Nov 2011 was 18% below pvgis.  May 2012 was 7.6% above pvgis.  May 2020 was 27% above.  Worry hat discarded for now.

Exiled_Tyke

EricMears said:

Can't manage 10+ I'm afraid but we have had them since 2011.
I was slightly worried to see that last two months have been below 80% of the pvgis suggestion but April & May were both >120% of it so relaxing somewhat.  Looking back,  I see that November is practically always below pvgis and this years 20% down is a lot better than last years 35% down.  Even Nov 2011 was 18% below pvgis.  May 2012 was 7.6% above pvgis.  May 2020 was 27% above.  Worry hat discarded for now.

Yep pretty much the same for me. Nothing to worry about for Oct & Nov, the weather and hence output were both terrible.  And as you say April and May were exceptionally good. Which just adds to the uncertainty. I doubt we'll ever get a clear picture of panel deterioration.  May be comparing the first 4 or 5 years with say years 15-20 may show some trends.  Let's all come back in another 10 years to compare our numbers! 

jimjames

Thanks both. That seems to be my thoughts too, I suppose the thing that has surprised me is how little variability there is with generation year by year. This year is a perfect example, it's turning out to probably be our best yet but great spring months have been offset by lower generation later in the year so although it's our highest it won't be by much and certainly not by the amount that April/May exceeded previous years.
We're well above PVGIS consistently but May 2020 was 25% above our 9 year average as well. In comparison it was 55% above the PVGIS estimate.

Martyn1981

You've reminded me of something I used to do, but had forgotten completely. The Met Office used to produce anomaly maps of the UK. These had shaded pixels to show how the various weather that month, compared to the longer term averages. For instance you might get 90% generation one month, then checking the solar map for that month (used to be a couple of months behind) you would find that your area was 80% to 100% of average. Or you'd get a great months results, and find the sunshine was in (say) the 100% to 120% range of long term average.
This was, and presumably still is, a good way to compare your generation each month/year to a figure that takes into account the weather variance for that period.

ASavvyBuyer

Martyn1981 said:

You've reminded me of something I used to do, but had forgotten completely. The Met Office used to produce anomaly maps of the UK. These had shaded pixels to show how the various weather that month, compared to the longer term averages. For instance you might get 90% generation one month, then checking the solar map for that month (used to be a couple of months behind) you would find that your area was 80% to 100% of average. Or you'd get a great months results, and find the sunshine was in (say) the 100% to 120% range of long term average.
This was, and presumably still is, a good way to compare your generation each month/year to a figure that takes into account the weather variance for that period.

Are these the maps you are referring too?:

https://www.metoffice.gov.uk/research/climate/maps-and-data/uk-actual-and-anomaly-maps

For us, this is already the best year so far since installation, with the rest of December still to go.

Regarding degradation, there does not appear to be any; or if there is it is very low, as for the last 3 years total output has been higher than the previous year.

Martyn1981

ASavvyBuyer said:

Martyn1981 said:

You've reminded me of something I used to do, but had forgotten completely. The Met Office used to produce anomaly maps of the UK. These had shaded pixels to show how the various weather that month, compared to the longer term averages. For instance you might get 90% generation one month, then checking the solar map for that month (used to be a couple of months behind) you would find that your area was 80% to 100% of average. Or you'd get a great months results, and find the sunshine was in (say) the 100% to 120% range of long term average.
This was, and presumably still is, a good way to compare your generation each month/year to a figure that takes into account the weather variance for that period.

Are these the maps you are referring too?:

https://www.metoffice.gov.uk/research/climate/maps-and-data/uk-actual-and-anomaly-maps

For us, this is already the best year so far since installation, with the rest of December still to go.

Regarding degradation, there does not appear to be any; or if there is it is very low, as for the last 3 years total output has been higher than the previous year.

Bingo, that's the site.
So I went to sunshine, compare to 1981-2010, I checked 2014 (108%) and 2017 (99%), and the map shows S.Wales in the 110-115% and 95%-105% range respectively.
Possibly not bullet proof, but I think it's a good way to check panels are performing normally. In fact I probably forgot, as the results were so boringly normal each month, I didn't need to keep checking.
Also worth looking at the temperature maps, as this will impact generation too.
As to panel degradation, I assume mine must have had some, but clearly not enough yet to be obvious, so after nearly 10yrs, it must be in the 0-5% range, so more than happy that they've performed better than expected ...... just like everybody else's.

Heizen

 If solar panels are not forcefully damaged the average lifespan of a solar panel is 40 years.

The above figure is based on replacing a solar panel when the efficiency drops below 80%. Most solar panel manufacturers carry an 80% efficiency guarantee for 25 years. 

The median average of efficiency degradation for solar panels by year is .5%. By these standards, you would approximately have to replace a solar panel every 40 years if nothing forcefully breaks them first. 

Can you compute the degradation rate? Yes, you can compute it yourself. For a 0.8% degradation rate, this means that when your solar panels reach the 2nd year, it will have an electricity output of 99.2%. After 25, years, it will be 82.5%.
For the projected output, you can determine it by multiplying the degradation rate with the number of years you have in mind. Then, you just subtract the sum from 100.

Martyn1981

Can I ask where you get the 0.5% figure from? I think what this thread is showing, is that little to no degradation is being seen so far, when 5% (after 10yrs) should now be starting to be obvious, and reflected in annual gen figures.

tim_p

My experience since late 2014 has shown a small increase in each of the past 3 years, with this year beating last already and still most of Dec to go. Weather variations are doing a good job of masking any degradation inherent in the panels. 

Worst year, 2017 - 3921kWh
Best year 2020 - 4172 kWh with some more to come, confident of breaking the 4200 total.

JKenH

I have 2 Identical strings of panels on my east roof linked to one inverter and another 2 identical strings on my west roof to another inverter. One string on the east roof consistently reads at least 10-15% lower than the other string, and on my west roof the difference is more variable. At the moment in very low light one string is about 40% down but nearer 10% when sunny. Overall my system is only operating at in the region of around 93% what it should be. 

I first noticed the problems in September but still made 102% of my PVGIS estimate that month then in October 87% of PVGIS and November 81%. The previous year, when I believe my system was working fine, the figures for the same 3 months were September, 103%, October 87% and November 73%. 

I am not suggesting there is degradation of my panels, rather some other issues. What I am trying to say, though, is the monthly variations in irradiance will mask even significant reduction in output due to degradation. It might be possible to compare system output annually with annual irradiance (if that is the correct term) for your region over several years to discern a trend but weather conditions can differ quite markedly locally so even that might not be accurate. 

One would expect some level of degradation over time so the fact that people are seeing the same or even better levels of generation several years after installation suggests it will be impossible, at least for some time to measure a trend from annual generation figures. 

I suspect the best way of checking would be to monitor peak output of the panels on a number of perfectly clear summer days and compare the same days year to year. Just don’t adopt May 2020 as a baseline. 😊