Microgeneration
Solar PV & battery storage regulations
A grid-tied solar or battery system is a generator connected in parallel with the DNO's supply. That triggers BS 7671 Section 712 (PV), Section 826 (energy storage), DNO connection under G98 or G99, and — for grant/export eligibility — MCS certification.
DNO connection — G98 vs G99
G98 — small installations
Single-phase systems ≤16 A per phase (≈3.68 kW), or three-phase ≤16 A per phase (≈11 kW), can be installed and connected first, then notified to the DNO within 28 days. This is the standard route for most domestic PV and small battery systems.
G99 — larger installations
Anything above the G98 threshold requires prior DNO approval — application, technical review, formal offer accepted before install. Timescales can run to several weeks; plan for it in your project programme.
BS 7671 requirements
- Section 712 (PV): DC string protection, string voltages within inverter MPPT window, dual-lockable DC isolator at the array, AC isolator adjacent to the inverter, dedicated AC connection to the consumer unit.
- Section 826 (energy storage): ventilation and thermal management, DC and AC isolation, protection against back-feed during isolation, clear labelling of energy sources.
- Loss-of-Mains protection: inverter must include DNO-approved anti-islanding (typically G100 or ROCOF/vector-shift settings) so the system cannot energise a dead grid.
- RCD selection: a transformerless inverter can leak smooth DC — a Type B (or Type A + DC-fault detection at inverter) RCD is usually required upstream.
- Labelling: dual-supply warnings at the origin, at the consumer unit, and at every isolation point.
MCS & Smart Export Guarantee
MCS certification is not legally required to install PV or battery, but it is required to claim under the Smart Export Guarantee (SEG) — the tariff licensed suppliers pay for exported units — and for the current 0% VAT rate on installation. Paul works alongside MCS-certified installers on projects where those benefits matter.