I use a cheap Harbor Freight, (about $15.00) solar panel to keep a 7 amp hour gelled battery charged, and it is always topped off putting out 12.6 volts even though I use my FX-4A hooked up to the same battery all day long. If you’re thinking of powering the radio directly from the solar panel, I wouldn’t, because the internal regulation would cause clicking noise in the receiver, and some panels in full sun light put out between 15 and 18 volts. In any case some kind of regulation between your panel and your radio/battery is needed.
Joel
KB6QVI