You'll have to verify any legal requirements, but the most useful electrical monitor is a unit that allows you to read voltage AND amps. If you are required to have a voltage warning light installed, consider a unit like the Electronics International VA-1A. It reads volts, amps, and has over/under voltage lights built in.
Traditionally the shunt for the ammeter was placed in the main bus wire. With a generator this made sense as when a generator stops charging, current from the battery will flow to ground through the generator itself, showing as current coming FROM the battery instead of INTO the battery (reverse current). If you have an alternator installed, I prefer to install the shunt in the alternator output lead. Alternators will not create a reverse current situation, if they stop charging, as they use diodes to control current flow and diodes only allow that flow in one direction. This shunt location will allow you to monitor total current flow from you alternator at any time, which is useful for troubleshooting. It also allows you to actually measure the current flow to individual systems by noting the amps reading before and after turning on a switch/breaker. For example, you show a reading of three amps on the ammeter. You turn on the nav lights and the ammeter shows five amps. Therefore your nav lights pull two amps. In the future, if you turn on the nav lights and the ammeter does not increase by two amps, you know at least one of them is inoperative.
As for normal monitoring of the charging system, I prefer watching the voltage. Over or under voltage conditions usually show up quickly and catching those conditions can keep you from burning out components or allowing you to turn off systems to preserve battery voltage for the trip home. If you only have an ammeter, how do you know what is causing a high or low reading?
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