Many boats these days have a battery or systems monitor permanently installed. Popular models include: E-Meter, Link 10, Victron BMV, Philippi BCM, etc.
With these meters, DC current is measured in and out of the battery by a device called a Shunt that is installed in the negative lead to the battery. A Shunt is simply a bar of metal with a known resistance between the two ends.
The resulting drop in current is then measured by the monitor and multiplied to give the correct current reading. The Shunt is the very last item connected to the battery negative post, and no other negative leads must be allowed to by-pass it. This is to ensure that it measures every amp of current going both in and out of the battery.
As you can see from the example below, it is possible for solar panels or a wind/hydro generator to supply power for DC loads directly, via bus bars or other connection points, without their current flowing into the battery or through the battery monitor Shunt.
Therefore, the Battery Monitor can only see and record what goes in and out of the battery, but not the different sources of those amps.
In order to monitor solar, wind, or hydro output, a dedicated monitor needs to be installed, with its Shunt connected in the negative output line of the charge controller, and "upstream" of the Battery Monitor Shunt.
The example below shows a solar application, but the same configuration can be used for any individual charge source or load that requires monitoring.
The Coastal Watt Wizard shown here will show instantaneous amps, volts, and watts, as well as accumulative amp/hours and run time, for any DC charging source or load.