Battery Monitors (shunts)
These are very simple to install! Simply disconnect your batteries main negative cable and slap this in between. Bam! You now know how much energy your batteries have available and you can track it over time. Every year or so you should use your shunt to do a full capacity test of your system to ensure it is working properly (affiliate links below)
This is the best one by far. I have a small box of these. Throw it on your system and connect with your phone. Worth every penny. Size it larger than what you need! It needs to handle the current of your entire system! Not just your inverter.
300- 2000A Victron Smart Shunt:
Budget Battery Meter
Dirt cheap bare bones monitor. I would still go with the shunt above due to the features, but if you are on a budget, this works fine. I never had an issue with it. No bluetooth.