If you look at the size of the battery in most all-in-one solar fencers it is very, very small.

A battery charger with a good high capacity deep cycle 12 volt battery is a whole different animal. Compare the joule output, its much higher on most 12v battery fencers compared to solar fencers.

You can try to keep the deep cycle battery charged with a solar panel but just make sure you get a panel setup with some type of charge controller. A standard roof top panel puts out 35-40 volts and can over charge a 12 volt car battery. You need a charge controller.  Overall system is not cheap.  With weeds and all contacting the fence wire I feel it takes 6 joules to hold cattle.