I need my EMU-1 powered up when motors are off to monitor a sullage tank so I have connected it to the instrument circuit breaker. I also sometimes run an engine to charge batteries and in that case I don't have the instruments running.

To power the EMU-1, I have 12v from the instrument breaker and 12v from each ignition switch all connected via diodes. This works fine, except that the measured voltage is of course out by the 0.6V (typical) voltage drop of the diode.

My suggestion is to add a calibration value to the measured voltage so that the voltage reported to the NMEA2K network is accurate.