http://www.apricus.com/html/solar_co...insolation.htm
Neat site with a clickable map, will give you monthly and annual mean solar insolation figures (in kWh per square metre per day) for all the major cities in your area. Values are derived from NASA and are 10 year averages.
For instance, Edinburgh gets an average of 2.26 over the year, peaking at 4.34 in June but dropping to 0.32 in December.
(I live a couple of degrees north of Edinburgh, so I'd be lucky to get 0.25
)