This year I purchased a GW5600J-1 and an MTG-910DA (both of which are solar powered), and in recent weeks both were down to a medium charge.
I placed them on my brightest window sill, but after a few days their charge level hadn't changed; with the onset of winter, days have been short and cloudy.
After reading the user manuals for the watches, and discovering the lighting requirements and the terminology for such things, I searched online for a suitable lamp.
I proceeded to discover a series of lamps/lights intended for people who suffer from seasonal affective disorder (aka SAD). Most models of such lamps produce 10,000 lux, which Casio's manuals refer to as "sunlight through a window".
I took the plunge on one relatively compact model: Philips goLITE BLU HF3332.
It can only stay on for an hour at a time. However, after approximately five 60-minute "sessions", both watches are now reporting a high charge level. The lamp does not get hot, so I don't have to worry about the watches getting heat damage.
Anyway, I was wondering how others with solar watches keep their rechargeable batteries topped up during darker months of the year.