Many vessels are installed with energy storage units in the form of batteries. The aim is so that they can generate a stable and secure electricity supply either alone (pure electric system), or in combination with one or more diesel generators (hybrid system).
However, the use of electricity generated onshore is not necessarily always the reason for installing batteries. In many situations, the idea is that the batteries will in fact only be charged using the on-board generators.
So you might be forgiven for thinking what’s the point of installing batteries if the motivation is not charging from onshore? However, under certain circumstances, it is possible to use batteries to reduce both fuel consumption and emissions, including in situations where all electric energy are produced on board.