There are two primary bits of information that indicate the status and rate of change of your electrical system, which is displayed on:
- Voltmeters, and
Voltmeters display of course voltage. In this application, the voltage is useful because it shows the level of charge of your battery. A 12V (nominal) battery will top off somewhere around 13.5V, and be fairly well discharged by the time it drops much below 11.5V or so. Variation within that rough range tells you the state of charge.
You can also use a voltmeter to indicate the output of your boat’s alternator. 14V or even 14.5V when the alternator is running would be normal.