To determine what size solar panel is needed to charge a 12V battery, there are a few factors to consider, including the battery capacity, expected discharge rate, and energy consumption. Here are the general steps to follow:

1. Determine the battery capacity in amp hours (Ah) or watt hours (Wh).
2. Calculate the expected discharge rate in Wh per day.
3. Divide the discharge rate by the battery voltage to get the required Ah per day.
4. Multiply the required Ah per day by the number of hours of sunlight available to get the required charging current in amps.
5. Divide the required charging current by the voltage of the solar panel to get the required panel size in watts.

For example, if you have a 100 Ah 12V battery and expect to discharge it by 50% each day (i.e., 600 Wh per day), you would need a solar panel that can provide at least 50 Ah per day (i.e., 6.25 A for 8 hours of sunlight). To calculate the required panel size in watts, you would divide 50 A by 12 V to get 4.17 W. However, its important to note that this is just a rough estimate, and the actual panel size needed may vary depending on factors such as weather conditions and the efficiency of the solar panel.