My AC has one built into it that keeps the battery topped off while the rig is not in use.
Basic requirements are you need a solar Cell (or as many as you can afford to match your power requirements) and a regulator that can handle the amount of output power produced by your cells.
If you don't use a requlator you can over charge your battery causing water loss.
I hear a company in Canada just invented a cell that uses UV rays, and is a new electonic design that should lower the cost 5X. But who knows how long it will take before we see them in the market.