I wanted the option of solar charging my 12V batteries; I found it cheap and simple.
All you really need is a panel to collect and convert the energy, and a controller to send it to the battery at a safe current for charging!
You can just buy a kit and get all the parts you need in one click. I’ve found, however, that shopping for your own panel, controller and wiring gets you a nicer and more reliable set for about the same price. I have also standardized on the same quick-connect pigtails my pulse battery charger uses (and I’ve installed on my motorcycles and cars,) so I like to just buy the parts.
I chose the 30W panel for its ability to nearly, fully charge my 400W battery in 8 hours of decent sunlight. Most often I use the panel to charge this inverter/jumpstarter/air compressor while camping. I’ve made it two weeks without needing to plug the battery in to a wall, limited only by available time to keep camping, not the efficiency of the charging system. Make sure to pick the right size panel for your application!
All in all, the solar charging of 12V batteries is pretty simple.