Assuming you have a lead acid battery, each cell can hold 50 Ah, or 600 Wh. A 12 volt battery has 6 cells for a total of 300 Ah, or 3600 Wh. If you solar panel is 18 volts, you will need 200 watts to charge the battery in 4 hours. In other words, you need a 100 watt solar panel for every 1200 watt hours, or 1 amp hour of electricity that you want to generate.