I’d be interested in the answer to this question also. I have been pondering a solar project myself.
A quick search indicates this product appears to be designed to charge solar panels. Unfortunately at the present time it is out of stock.
Update
Ah, I just found stocks of the PiJuice Zero - my recent searches indicated these are hard to come by, but at this link they seem to be in-stock. The maker claims this will work with a solar panel power source.