What is the difference between a sunny day and a cloudy day? Up to four times the amount of solar electric power!!
Here is a comparison of two days in August with very different weather. On August 13, 2013 there were clear blue skies all day. Two days later on August 15, there were dark clouds and rain all day. Although the day length was almost identical, the solar production was dramatically different, as this graph below shows.
The clear-sky sunny day produced 21.4 kWh of power, with peak production of almost 2800 watts. The rainy day with thick dark clouds 2 days later produced only 5.1 kWh — about one quarter of the power — with peak production of 1100 watts. Not every rainy day is as dark as it was on the 15th and even then there was decent power production, but clearly clear skies makes a big difference.