There are plenty of things on a sailboat that use energy in one way or another. That energy passes through the boat systems by way of electrical power when connected to a shore power outlet or is “drawn from” batteries that act as a source of electrons to operate boat systems dependent on electrical power.
I employ AGM batteries on Skiron. There are four “house batteries” and one starter battery in place. They are coupled in a way to provide the maximum amount of output.
The batteries can be charged and maintained by connecting the battery charger system to shore power via a cord that plugs into a shore power outlet and into a receptacle on Skiron. They can also be charged by the alternator attached to the diesel engine when the engine is running. Skiron has three solar panels that are able to charge the batteries at the dock or while sailing with the engine off. The vessel is also fitted with a wind generator that turns in the apparent wind and charges the batteries while sailing or while docked when the wind is blowing in the harbor.
Redundancy is essential when it comes to supporting the electronics and other systems dependent on electrons as a power source.
Shore power receptacle 
Alternator 
Port side solar panel 
Wind generator
The electrical system on Skiron is rather complex. I will write about other aspects of the design and some unique features in the future.
Sailing Skiron 