You need to confirm load first.
If battery completely off then very little needed.
If parasitic loads like alarms present then 3 times their load to recover from night time use plus battery charging.
Focus on power needs first and confirm battery charging requirements then select a solar controller that meets that need and a panel twice the size.
Dash board plug into cig lighter not good as that connection is off if battery disconnected usually.
Also note windshields usually have some uv filtering to make driving more pleasant but that reduces output from panels placed on dash.
Also requires dash in sun and better to park other way to keep in shade to reduce sun damage.
Look at panels on roof via top of ac units.
Tony & Lori
1989 Country Coach Savannah SE