The most important instrument is the battery monitor
. Knowing the net battery
AHrs in/out is the most vital information. Some monitors support more than one shunt so you can get information about solar
However, a more sophisticated solar
controller with a digital display, as Mark says, is generally the best way to detailed information about the solar output. The advantage over a simple ammeter is that you get an AHr counter for solar as well as A. In other words you can see the total output as well as the instantaneous production.
The next level is to integrate the battery monitor
and solar regulator
. This is overkill for most boats, but the communication between these two instruments allows for a more accurate charging
ebay meters are still great and can be easily modified to read voltage,current, temperature etc.