Lets assume your battery is of 12 volt and 20 ampere. Then it will take 5 hours to charge battery of 100 ah. Then you will require 240 watt solar panel to be exact. Our opinion for you is to buy 300 watt solar panel single or 3 x 100 Watt Solar Panel. If you it single then you need not assemble it every time but if you buy it separately then you can use it multi purposely.