I think that mBuino is great and its low power consuption is awsome. But i think its outrageous to not have a solar charging option.
There are so many cool flexible solar panels running 0.8-1.5w @ 0.5-2v(not in this order) but I haven’t seen any ready-to-buy circuit to boost it up to a 3.3v with a backup battery. ITS OUTRAGEOUS !!!
We just used a boost regulator to get 3.3v out of lower voltages. I will dig the part number out once at the office.
And welcome to the community 
"0.8-1.5w @ 0.5-2v"The conclusion certainly sounded is good 