This is a common misconception about solar. Even when it is overcast solar panels work just fine. They don't produce quite as much, but they work. In fact on a heavily overcast day you get about 80% as much energy as on a clear day.
My experience has been you get about 10-20% of the energy out of a cell, while the sun is blocked by cloud.
NASA has some data about the average solar irradiation for every spot on earth, it is quite useful. It is in a spreadsheet.
It is measured in hours of direct sunlight per day and obviously depends on the time of year. ie no cloud.
I think here we get 6 hours per day in summer and 3 in winter.
The rain is not directly the issue, but it sounds like the lack of sun is.