Time to charge = (Battery Voltage x Battery Ah )/ Solar Panel Watts Theoretically, if you have a 100 AH battery with the sun shining directly on your panels, it will take (12x100)/100 hrs = 12hrs. Because the sun is not always directly overhead, and due to other inefficiencies, multiply the result by 2 or 2.5. This means it will take about 24 - 30 hrs of direct sunlight to charge a fully depleted 100Ah battery completely. If we estimate 5 - 6 hours of sunlight in a day, we can expect this solar panel to charge your 100Ah battery in 5 - 6 days. Therefore, you will need 4 of these panels to charge your 100AH battery in one day.