You can view a month by month comparison of solar PV electricity output here
And a year by year comparison of solar output
We have a tree in a neighbouring garden that shades the solar panels when the sun is low around the shortest day. From approx 2pm onwards the sun starts to be blocked from some panels which dramatically reduces the electricity generated. The tree may be higher that in previous years which would cause more blocking effect. You can see the impact of shading on the roof from the trees as below.
|Dull November Means Poor Solar Electricity Generation|
The long range forecast for December 2013 is for more sun and colder days which should improve the generation.