Charging a 45A battery typically requires around 540 to 720 watts for effective charging, depending on the state of the battery and solar panel efficiency. Use our solar panel size calculator to find out what size solar panel you need to charge your battery in desired time. Simply enter the battery specifications, including Ah, volts, and battery type. Solar panels generate direct current (DC) electricity from sunlight. When charging. To determine how many solar panels you need for battery charging, consider these steps: Identify Your Energy Consumption: Calculate how much energy your devices consume daily, typically measured in kilowatt-hours (kWh). Whether you are powering a cabin, RV, or backup solar system, understanding.