EDIT: I used the wrong solar cell in this experiment, using a single solar cell 0.5V the supercap charges fully for less than 45 min
I wired my LTC3108 breakout board this morning to a 120x80mm solar cell, shamelessly scavenged from a perfectly working solar Christmas decoration. I punched in a 1F (whoa) super capacitor rated 5.5V and left this setup by the window (south facing). It was snowing almost all day today and it was completely overcast. The solar cell measured (under this load) between 30 and 100mV depending on how cloudy it was at the moment – that is ideal for the LTC3108 that has a minimum 20mV start up. Seconds later I got the 2.20V on the VLDO and a minute later – Power Good and 3.3V on the V-OUT. This module is intended for short bursts of usage, just as the Funky is doing anyway. The storage capacitor managed to add 1.4V to its initial voltage during the daylight and now measures 3.5V, it will probably take 2-3 more days to fully charge it with no load and if the current cloudy conditions continue. Nothing comes free of course, the efficiency in my setup is only 30-40%, but still making usage of these low voltages is worth it. As far as I am concerned, this is the solution for solar powered remote nodes.