To find out what size solar panel you need to charge your battery, you'll need to enter the following info into our solar panel size calculator at the top of this page: 1. Battery Voltage (V):What is your battery's voltage? 2. 12 volt batteries are the most common voltage I see people using in their solar power setups. Here is a chart showing what size solar panel you need to charge 12V batteries of vario. 12V 100Ah batteries are some of the most common in solar power systems. Here are some tables with the solar panel sizes you need to charge them at various speeds: The 12V 50Ah battery is another common battery size in solar power systems. Some car batteries are also 50Ah. Because lead acid batteries only have 50% usable capacity, a 50Ah. Because lead acid batteries only have 50% usable capacity, 200Ah lead acid batteries have as much usable capacity as 100Ah lithium iron phosphate batteries.