There seems to be no little light any more. (The four LEDs on the power bank work like the candles of an Advent wreath: one fewer for each quarter of charge lost.) My usage statistics point steeply downward; currently I am under an hour a day, less than half of before.
The power bank gets no sun at all here, and most of the time it has to stay indoors because I don't want to leave it alone in the rain (or the snow, they alternate here). Since I can't measure anything on the gizmo, I got hold of a larger solar panel that should deliver about 19 watts in sun. This morning the maximum current I could draw at 5 V was 20 mA. So a rather lousy 100 mW instead of 19 W. That clearly means that winter here is really hard. I still have half a phone charge; I'm hoping for tomorrow…