Sun

This chart below shows the level of sunshine for our area (yellow), according to the Met Office's anomaly / actual data - this compares the previous month to that month's average over the last 30 years.  So in Jan 2012 we had about 40% more than of the average level of sunshine for January.  This should give us an idea of whether that month has seen an exceptional amount of sunshine when viewed on an average basis.

The green columns show our generation figures compared to the PVGIS predicted level for our system (30˚ slope, facing WSW - 195˚, 4kWp array, 3.6kW inverter, West Yorkshire).  So in Jan 2012 we generated more than 50% above the expected level for our system, thanks to 40% extra sunshine!

February and March 2012 have been above par thanks to unusually high levels of sunshine, although April - August have been more or less on the average levels for those months.  Let's hope September is another March!

monthly solar PV performance (green) vs metoffice sunshine stats (yellow)

* please note that the sunshine figures can be misleading because they refer to a percentage of the expected or average sunshine level for that month, not to an actual number as the PVGIS figures do.
Therefore the benefit of January 2012 being 40% above average is probably much smaller than the disadvantage of June being 40% below average.  
This is because the average / expected level of sunshine for January is a fraction of June, not only due to the weather factor but the length of the day (15th January - 8 hours; 15th June - 17 hours).