20W = 2x 10W panels.
Here = Hong Kong where we have an average of about four hours of sunshine throughout the year but sometimes weeks of cloudy/rainy weather which indeed does drain the battery at times, but it's quite rare. A bigger battery would solve that; often enough I see the green "battery full" LED on, so I'm wasting energy.
I could anyway do better by placing the panels in a place where they actually can catch all the available sunlight (until noon they're in the shade of a wall), and where they're facing towards the direction they can see the sun instead of slightly towards the wall (which is to try and have rainwater drain off of them). That could get me a lot more power out of them. Having the panels flooded (you may take that literally, I've had them submerged) at times of heavy rain is of course also not exactly ideal.