There are numerous energy storage technologies that can be used to store energy from variable sources such as photovoltaics. These include batteries, such as the lithium-ion batteries used on board the solar ship PlanetSolar, and hydrogen (as used on the Viking Lady demonstration cargo ship). The issues with batteries are with regard to weight, maintenance and safety. It is important to be aware that batteries and fuel cells can operate for a limited amount of charge/discharge cycles. Therefore one of the requirements of the control system for an electric cargo ship such as that proposed within INOMANS<sup>2</sup>HIP is to limit the number of cycles per voyage.