Estimate how long it takes your solar panel to charge a battery based on panel wattage, battery capacity, voltage, and charge efficiency. Formula: Charging Time (h) ≈ (Battery Ah × V × (Target SOC / 100)) ÷ (Panel W × (Eff% / 100)). Adjust for sunlight hours to find daily charging duration. There are many different variables that will affect the ultimate result, such as the size of the battery, the efficiency of the panel, the number of hours in a day of sunlight, etc. Optional: If left blank, we'll use a default value of --- 50% DoD for lead acid batteries and 100% DoD for lithium batteries. I'll run through each method step by step, starting with the simplest and ending with.