We live near St. Davids in Pembs. and below are some data for our 3.9 kWp solar PV array installed last April.

The first column shows the average number of sunshine hours for the month (taken from Met Office data from the Aberporth weather station from 1942 to 2010), the second the actual number of sunshine hours for 2011 and the third the number of kWh generated by the PV array.

Apr 172 223 386
May 212 194 405
Jun 207 234 491
Jul 190 206 412
Aug 177 142 333
Sep 139 127 253
Oct 101 73 140
Nov 61 73 78
Dec 50 25 33

As I suspected December was very cloudy; in fact there were only 25 hours of sunshine for the whole month. The Met Office website states that these are preliminary data but these usually prove to be quite accurate. If these are correct then it will be the lowest figure recorded in nearly 70 years!

There are no data for January (2012) yet but as we only generated 54 kWh in the month, then I suspect that we will find that we got only about 50 hours of sun at best.

Incidentally a graph of output vs sunshine hours gives a reasonably linear plot (correlation coefficient = 0.95).

I would be interested to know what sort of outputs others achieved, especially in this neck of the woods! With a bit of luck we should hit 3000 kWh by end of March (i.e. 12 months "worth").

Posted By: Jeff BWe live near St. Davids in Pembs.
[SNIP]
I would be interested to know what sort of outputs others achieved, especially in this neck of the woods!

There are lots of examples in the Monthly PV production threads in the 'Electricity Generation/Consumption Data Logs' section of the Navitron Forum, e.g.
http://www.navitron.org.uk/forum/index.php/topic,16413.msg186676.html#msg186676
As of a few moments ago the latest post in both December and January topics was from someone in Cwmbran (not exactly local to St Davids, but at least in S. Wales).
The above link should go direct to the January Cwnbran post.

Posted By: skyewright

Posted By: Jeff BWe live near St. Davids in Pembs.
[SNIP]
I would be interested to know what sort of outputs others achieved, especially in this neck of the woods!

There are lots of examples in the Monthly PV production threads in the 'Electricity Generation/Consumption Data Logs' section of the Navitron Forum, e.g.
http://www.navitron.org.uk/forum/index.php/topic,16413.msg186676.html#msg186676
As of a few moments ago the latest post in both December and January topics was from someone in Cwmbran (not exactly local to St Davids, but at least in S. Wales).
The above link should go direct to the January Cwnbran post.

Skyewright - many thanks for the link. Wouldn't have thought of looking at the Navitron website for this sort of info. Yes, the guy in Cwmbran has pretty low PV generation readings for both Dec and Jan, but even with panels half facing due south and half facing due west his are still better than ours which are all facing SSE! In fact reading all the other posts on that thread make for quite depressing reading - some of the outputs are quite incredible! Having said that, for the last 2 days we have generated almost 10 kWh per day, having enjoyed clear blue sky on both days.

Posted By: SteamyTeaThink you will find (with a bit of x-axis transformation) that the distribution fits f(z)=(1/(2*Pi)^0.5)*e^-((z^2)/2)
or it is a normal, cumulative distribution with a mean of 64 (for Jan in Cornwall) and a Standard Deviation of 1
Should not come as a surprise as we know the sun rises every morning like clockwork and sets when we expect it to, the weather, over 4 weeks or so, is pretty predictable (though may not seem so to us as we remember the last extreme).

SteamyTea - thanks for your response. Sorry to sound a bit thick, but are you saying that the number of hours of sunshine for Cornwall this January was 64?

Ed Davies - thanks for your response. Isn't there is an amazing variation in output figures? If only more posters would say the actual place (or even just county) where they are located. All this coordinates stuff - let's have it in old money please!

Posted By: Jeff BSteamyTea - thanks for your response. Sorry to sound a bit thick, but are you saying that the number of hours of sunshine for Cornwall this January was 64?

No, thatis the number that is used to transform the x-axis to make it fit a normal cumulative distribution. A NCD has a mean of zero and a SD of 1, so it has to be offset and multiplied or divided. Not something you work out in your head but use a program that allows curve fitting. I used Excel for this though.

Posted By: Jeff BHaving said that, for the last 2 days we have generated almost 10 kWh per day, having enjoyed clear blue sky on both days.

At this time of year a few hours of blue sky can make a lot of difference.
January 30th alone provided ~10% of our January generation.

This little table gives a rough idea of the highs & lows of production for 2011 for our system (WSW facing, 42 degree slope, ~57 degrees lat).

Overall% => Overall generation cf. PVGIS longterm monthly average.
Best% => Best day cf. PVGIS longterm daily average for month.
Worst% => Worst day cf. PVGIS longterm daily average for month.

Overall% Best% Worst%

Jan 2011 127 512 7
Feb 2011 106 330 3
Mar 2011 91 270 6
Apr 2011 99 214 4
May 2011 90 187 8
Jun 2011 111 206 11
Jul 2011 134 219 19
Aug 2011 110 217 18
Sep 2011 116 291 6
Oct 2011 82 305 5
Nov 2011 113 390 0
Dec 2011 62 200 0