In series: the operating voltage output adds up, while the system current output is the same as that of one panel.
In parallel: the operating current output will add up, while the system voltage output will be the same as the output of one solar panel.
To achieve optimum power output and system rating for your power system, you can use both parallel and series connections.
Power is measured in watts, and the capacity is commonly measured in Watt-hours (multiplying power output in watts by the required number of hours of operation multiplied by a safety factor of 1.5-2). Therefore, it is wise to size your battery bank first based on the minimum required capacity, and then accordingly, decide how much input power and how many panels are needed.