Hi, somewhere there is reference to a 50W panel for our latitudes (and weather!) although in the thread above perhaps 20W is thought sufficient. Panels here in mid winter produce < 4% of their rated output and a number of dull short days in a row almost nothing.
I am only a few degrees North of you. I confess though, that I use a 2A 12V supply (not sure of actual output probably 13V+) and a 15m+ long piece of old lawnmower wire carrying 12V , nice and thick to reduce resistance and voltage drop. Both TX units work fine with this. (TX Units currently showing 12 and 13 volts at TX board.)
I am only a few degrees North of you. I confess though, that I use a 2A 12V supply (not sure of actual output probably 13V+) and a 15m+ long piece of old lawnmower wire carrying 12V , nice and thick to reduce resistance and voltage drop. Both TX units work fine with this. (TX Units currently showing 12 and 13 volts at TX board.)