The capacity of your 12V battery combined with how quickly you want to discharge it will dictate the size of the solar panel you need. As a general recommendation, a 100W solar panel is a good starting point for charging a 12V battery. It can produce around 30-40Ah per day under optimal conditions, which is suitable for basic off-grid setups. If you’re using more power or need faster charging, consider using a larger panel or multiple panels.