The size of the solar panel you need depends on the capacity of your 12V battery and how quickly you want to recharge it. As a general guide, a 100W solar panel is a good starting point for charging a 12V battery. It should be able to deliver around 30-40Ah per day in good sunlight, which is suitable for most basic off-grid setups. If you’re using more power or need faster charging, you may need a larger panel or multiple panels.