To charge a 12V 20Ah battery, you need a solar panel of at least 100W. Consider adjusting the panel size based on charging time, sunlight hours, and your energy usage for. Use our solar panel size calculator to find out what size solar panel you need to charge your battery in desired time. Simply enter the battery specifications, including Ah, volts, and battery type. Assess Daily Energy Needs: Calculate the total wattage of devices you intend to power to choose a solar panel that meets or exceeds this daily. To determine the appropriate wattage of solar panels for a 20A battery, several factors come into play. Battery capacity is essential; a 20A battery typically denotes a 12V system, which means its total capacity is 240 Wh (watt-hours). Optional: If left blank, we'll use a default value of 50% DoD for lead acid batteries and 100% DoD for lithium batteries. If you get 6 hours of sunlight daily, a 100W panel provides enough energy.