DNO G99 & G100 Grid Connection Guide for Commercial Solar

The Energy Networks Association (ENA) sets the connection criteria for distributed generation. While domestic arrays under 16A per phase connect under simplified G98 notify-and-fit rules, commercial solar installations (>11.04kW on three-phase) fall under Engineering Recommendation G99. This requires a formal technical application submitted to your regional Distribution Network Operator (DNO) before any hardware is energized.

Distribution infrastructure in Great Britain is managed by six regional DNOs across 14 licence areas: National Grid Electricity Distribution (Midlands, South West, South Wales), UK Power Networks (London, South East, East), SP Energy Networks (Central & Southern Scotland, North Wales, Merseyside), Scottish and Southern Electricity Networks (North Scotland, Central Southern England), Electricity North West, and Northern Powergrid (North East, Yorkshire). In constrained network areas, DNOs may initially quote significant reinforcement fees for unlimited export.

When a DNO indicates that the local high-voltage or low-voltage network cannot absorb your system's peak generation, installers specify an ENA-approved G100 export limitation device. G100 systems continuously measure power flow at the grid boundary point. If on-site consumption drops below solar output, the G100 controller instantaneously throttles inverter generation within 5 seconds, ensuring export never exceeds the pre-agreed DNO threshold. This allows businesses to install 500kWp+ systems behind the meter without paying £50k–£250k+ in utility network reinforcement charges.

1. **Preliminary Network Study & MPAN Review (Week 1–2)**: Review half-hourly import/export data, Maximum Import Capacity (MIC), and local DNO substation headroom capacity. 2. **Formal G99 Standard Application Submission (Week 3)**: Detailed single-line diagram (SLD), protection settings, inverter type-test certificates, and G100 schematic submitted to the DNO. 3. **DNO Assessment Window (Weeks 4–12)**: Statutory 45 to 65 working day evaluation period where DNO electrical engineers model fault level contribution and thermal capacity. 4. **Connection Offer Acceptance**: Review connection terms and export capacity allocation. 5. **Commissioning & DNO Witness Testing**: Certified commissioning and protection relay witness testing prior to permanent commercial operation.