The SolaX X1-FIT AC Coupled 3.7 kW Charger is a high-efficiency retrofit battery inverter designed to add solar battery storage to existing single-phase PV systems while enabling smart energy management. It supports AC-coupled charging and discharging with up to ~97 % efficiency and offers flexible operation modes including self-use, feed-in prioritisation, and backup (EPS) capability. The unit features rapid EPS switchover (<10 ms), CT compatibility for intelligent load response, VPP readiness for grid services, and robust safety protections in an IP65-rated enclosure. Ideal for residential energy storage upgrades, it integrates smoothly with a wide range of high-voltage battery systems and supports enhanced system control via remote monitoring.