The battery charging and discharging panel and the uninterruptible power system (UPS) are important electrical power sources for ships, which mainly supply power to automation, communication & navigation and safety systems. The specified discharge time can then be achieved to maintain the emergency operation, depending on the sufficient energy storage and the hot standby of the battery set. The required battery capacity is accurately calculated through the analysis of the discharge characteristics, capacity factor and the calculation method of the battery. It can not only ensure the safety and reliability of the electricity consumption of ships, but also improve the economy of the selection design. It mainly analyzes the capacity calculation methods for the common marine battery under several common occasions. It can provide guidance for verifying the selection of the battery capacity, and improve the ship progressiveness with more safe, reliable, economic and environment-friendly configuration of the power station.