Many people use batteries with solar panels to recharge the batteries. It usually doesn't take a whole lot of solar to keep a fence charged. My experience says between 5 and 10 watts is usually plenty.
I guess unless you have a cow that really really likes a little voltage in the morning. If you don't then you will probably be OK with 5 or 10 watts. If you do have a pesky voltage hog, then you might need a larger panel. You will definitely need a charge controller on 10 watt panels and above. You don't want to ruin your batteries by overcharging them.
So for all your Electric Fence Solar Battery Chargers check out OutsideSupply.com