How to make a Solar-Powered Slug
I had a 12 V, 4.8 W (nominal) solar panel doing nothing useful and decided to try to power my slug from it. So far it has not been very sucessful. Maybe readers who live somewhere with more sunshine than I get here in England will have more luck. Anyway, it is all described at http://chezphil.org/solarslug/.
Solar power is usualy very unstable, it's better to use the solar panel as a battery charger, and use the batery to run the unit itself. There are several power control ic's that can do this for li or mhi bateries. There are power control ic's wich will swicth between two or more power sources depending on conditions. I suggest that you can use one standard li cell and a dc-dc conververter to power the unit (this will be small enough) and use the solar source as primary source for the power control ic.