2011-12 sunlight data and performance guarantee analysis (West Yorkshire)

Analysis of sunlight hours data shows that solar PV systems in West Yorkshire installed after June 2011 will have suffered from reduced first year sunlight levels compared to the 1998-2010 average our predictions are based on.

This years unusually bad summer is largely to blame for this with sunlight hours around 40% below average in June and April, as well as July and August being 10 and 14% below average.

Most of the last 12 months were below average though, with March being the notable exception at 48% above average.

bradford sun hours data

Comparison of 2011 & 12 Monthly Sunshine Hours Data for Bradford, West Yorkshire


Sunshine data for first 12 months from month of installation

The data illustrates the annual variability of the solar resource though, as 2011 was 10% above average, with the 12 months from the end of March 2011 being 13% above average.

From June 2011, installations will have seen lower than average numbers of sunshine hours for the first 12 months of operation, although the data is incomplete for installations after September 2011, and a sunny Autumn could improve the situation.

There's no reason to think that this years lower sunlight levels should be anything other than an anomaly, with levels as likely to be above average next year as they are to be below it.


Month of installation 1st year sunshine hours Percentage of 1998-2010 average Performance Guarantee payment point comments
1998-2010 average 1325.3
Jan-11 1464.7 111% 95%  
Feb-11 1473.7 111% 95%  
Mar-11 1503.2 113% 95%  
Apr-11 1383.3 104% 95%  
May-11 1405.2 106% 95%  
Jun-11 1295.7 98% 93%  
Jul-11 1260.7 95% 90%  
Aug-11 1225.7 92% 87%  
Sep-11 1083.8 90% 85% 11 months data only
Oct-11 995.4 90% 85% 10 months data only
Nov-11 947.6 91% 86% 9 months data only
Dec-11 915 91% 86% 8 months data only
Jan-12 858.7 90% 85% 7 months data only
Feb-12 805.1 91% 86% 6 months data only
Mar-12 643.6 90% 85% 5 months data only
Apr-12 556.8 88% 83% 4 months data only
May-12 358.7 78% 73% 3 months data only
Jun-12 264.5 88% 83% 2 months data only
Jul-12 126.2 86% 81% 1 months data only
Aug-12       no data


Performance Guarantee

Leeds Solar offer a performance guarantee to our customers that applies to any systems that under perform our estimates by 5% or more in the first year of operation, allowing for variations in annual insolation levels.

We currently base our analysis of the impact of varitions in insolation levels on the sunlight hours data from the Bradford Met Office station, compared to the average for 1998-2010 (the same period as the PVGIS prediction tool uses for its base data).

The table above outlines the percentage below our original annual performance prediction at which Leeds Solar will honour our performance guarantee for customer, depending on which month customers had their systems installed.

The performance guarantee gives a 1% rebate on the initial purchase price for every 1% the system has underperformed below this level, and we will also double check your system to try to understand and remedy the cause of any under performance if possible.



Note, that sunshine hours data is not a perfect method of comparing actual light levels over the year, as it doesn't take account of the lightness / darkness of any cloud cover. This year's cloud cover is likely to be a lot darker than average as indicated by rainfall levels that are around 150% higher this summer than last.

We're trying to obtain more accurate insolation data, but will use the sunshine hours data as a reasonable approximation until more accurate data is available to us.

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