The battery inverter sits squarely in the centre of an off grid system. This makes it the perfect piece of equipment to document how much power is going in and out of the batteries. It can also document the state of charge and many other readings. This information gives you a clear understanding of how your system is operating. Also, referring to this data can help claim warranties, fix faults or adjust settings.
Make sure you get a battery inverter with a high level of monitoring and data logging.