BS 7671 Amendment 4: Solar PV Battery Systems

Solar PV systems with integrated battery storage are now the default specification for new domestic and commercial solar installations. Amendment 4 creates a clear regulatory framework by linking the existing Chapter 72 (Solar Photovoltaic Power Supply Systems) with the new Chapter 57 (Battery Storage Installations). Previously, the battery element of a solar-plus-storage system fell into a regulatory gap.

The interaction between Chapter 72 and Chapter 57 depends on the system topology. For AC-coupled systems — where the PV inverter and battery inverter are separate units connected on the AC side — the PV array wiring follows Chapter 72 and the battery circuit follows Chapter 57 independently. The key interface point is the AC distribution board, where both sources connect.

DC-coupled systems, where the battery connects to the PV array via a shared DC bus and a hybrid inverter, present a more complex compliance picture. Chapter 57 requires that the DC wiring between the battery and the hybrid inverter meets the isolation, labelling, and protection requirements specific to battery circuits. Meanwhile, Chapter 72 continues to govern the PV string wiring up to the DC combiner. The hybrid inverter itself must satisfy both chapters' isolation requirements, which typically means separate DC isolators for the PV strings and the battery, even where the inverter has a shared DC bus internally.

String voltage considerations change when batteries are in the circuit. Chapter 72 limits PV string open-circuit voltage based on inverter ratings and conductor insulation. When a battery is DC-coupled, the battery voltage adds to the system DC voltage under certain fault conditions. Chapter 57 requires that this combined voltage scenario is considered in the cable insulation rating and the DC isolator breaking capacity selection.

Anti-islanding protection must account for the battery's ability to sustain an island independently of the PV array. While the inverter's G99-compliant anti-islanding function handles this at the grid interface, Chapter 57 additionally requires that the battery can be independently isolated from the installation without disabling the PV system, and vice versa.

ECalPro's Solar PV Calculator now incorporates Chapter 57 battery circuit requirements alongside the established Chapter 72 PV sizing methodology, providing an integrated design tool for solar-plus-storage systems.