Typically, an off-grid inverter system with a 3 kW capacity could take 5 to 10 hours to fully charge the batteries, depending on how much sunlight is available and the load demand. Unlike string. To charge using photovoltaic solar energy, typically, the process requires between 1 and 8 hours, depending on several factors such as 2 the solar panel efficiency, 3 battery capacity, and 4 available sunlight. Formula: Charging Time (h) ≈ (Battery Ah × V × (Target SOC / 100)) ÷ (Panel W × (Eff% / 100)). Adjust for sunlight hours to find daily charging duration. Let's break this down with real-world examples and industry insights. Sunlight Availability: A 300W panel under full sun (1,000 W/m²) generates ~1. These times are based on the battery being charged at the maximum power output of the inverter.