Well, a solar panel with 50WP which costs about 41EUR with shipping on ebay, gives you 2,76A at 12v at full direct sunlight. Of course you need a 12v to 5v converter for that.
Additional, if you put it behind a window, it produces less then direct outside.
The 50WP should be able to output 200-500mA to power 1-2 even if its very clouded.
It could power far more in direct sun light but thats not always the case, thats why they include batteries.
The bigger the panel is, the more it provides also while its clouded.
But no math has been done here, I had a solar setup I knew a bit from that but thats it.