I currently have 2 solar collectors and play on Normal difficulty. I noticed that my solar collectors did not generate the same amount of income everyday. Sometimes I got 3000€, sometimes it's only 1000€. So, does weather affect solar collectors?
I think I found the answer. Go to Finances page and look for Property Income. There goes all of your income from properties (solar collectors, greenhouses, etc). I just did an experiment and no matter what condition is the weather, it always generate the same amount of income for the same number of solar collector. In my case, I have 5 solar collectors and I got around 11k€ everyday from Property Income.