My DFARs has been running 24/7 for the past 2 weeks.
Still design in progress, but at this stage it's just 3 simple modules involved; It will improve with planned changes.
Li-Ion charge module to charge a pair of 18650's in parallel with a 4.5W panel (6V & 165x165mm).
A "Juice Bank" module to boost the Li batts to a 5V output, which steps down again with an LM2596 Buck regulator.
Last 4 days have been overcast, but I have still not seen the battery's drop below about 75% charge of a morning.
Will see what to-morrow brings.
The regulator to the fan is set at 2.1V.
Cheers.