Beyond the G98 Threshold: When Notification Becomes Application
The Energy Networks Association's Engineering Recommendation G98 permits a single micro-generator of up to 3.68kW per phase (16A) to connect to the low-voltage network under a deemed right to connect, subject only to a post-commissioning notification to the Distribution Network Operator (DNO) within 28 days. This threshold is not arbitrary: it is the point at which a single type-tested inverter's fault contribution and export profile can be assumed, without site-specific modelling, to sit within the thermal and voltage tolerances of a typical domestic service cable and local distribution transformer. Any installation that exceeds this envelope — most commonly a multi-inverter array combining a solar PV inverter with a separate AC-coupled battery storage inverter, where the aggregate installed capacity per phase exceeds 3.68kW — forfeits the G98 deemed-connection right and falls under Engineering Recommendation G99, which mandates a prior-to-connection application and DNO assessment before any generating plant may be energised in parallel with the network.
The regulatory distinction exists to manage three interlinked grid physics problems on the low-voltage radial network. First, voltage rise: as embedded generation exports power back along a feeder, the voltage at the point of connection rises in proportion to the feeder's impedance and the exported current, and DNOs must keep this within the statutory ±10%/-6% envelope defined under the Electricity Safety, Quality and Continuity Regulations (ESQCR) even at minimum-demand, maximum-export conditions around solar noon. Second, phase unbalance: single-phase generation and storage connections are rarely distributed evenly across the three phases of a local distribution transformer, and uncoordinated high-capacity single-phase export can push neutral current and phase voltage differentials beyond the transformer's design tolerance. Third, thermal asset protection: distribution transformers and service cables are rated for a defined thermal envelope, and the DNO's assessment exists to confirm that the cumulative embedded generation capacity on a given feeder, including the new application, will not drive sustained loading beyond that rating during peak mid-day export windows.
The ENA Connection Framework: Application Pathways Compared
Application Pathway | Capacity Threshold | Approval Type | DNO Assessment Window | Typical Network Impact Fee |
|---|---|---|---|---|
G98 Notification | ≤ 3.68kW per phase (16A) | Deemed right to connect (post-commissioning notification) | 28 days post-install | £0 |
G99 Form A (Fast-Track) | > 3.68kW to ≤ 7.36kW per phase, with integrated export limitation | Fast-tracked conditional approval | 10–20 working days | Minor administrative costs (£0–£200) |
G99 Form B / Standard Full Application | > 7.36kW single-phase, or larger three-phase installations | Full system network modelling required | 45–65 working days | Variable — can trigger localised transformer or cable reinforcement costing £1,500–£10,000+ |
Engineering the Submission: Export Limitation and Topology Controls
A well-engineered G99 Form A submission is structured to demonstrate to the DNO's assessment engineer that the installation's as-exported profile, not its raw generating capacity, is what determines network impact. Three techniques dominate a fast-track-compliant design:
- Fail-safe active export limitation (AELS): An ENA type-tested export limitation scheme uses a smart meter or current transformer (CT) clamp at the grid origin point (the incoming supply head, ahead of any generation) to measure real-time import/export and dynamically throttle inverter output — typically within milliseconds — if cumulative export threatens to breach the agreed DNO ceiling. This allows, for example, 15kW of raw behind-the-meter generation and storage capacity to be installed while the DNO agreement caps grid export at 5kW, with the scheme's fail-safe behaviour (defaulting to zero export on communication loss) being the specific design feature DNOs require evidence of before granting fast-track approval.
- Managing the power factor envelope: Configuring inverters to operate dynamically across a power factor range — typically 0.95 leading to 0.95 lagging — allows the installation to absorb or inject reactive power in response to local voltage conditions, actively damping localised voltage rise on the feeder without requiring a reduction in real power export capacity.
- Split-inverter staging architecture: Separating solar generation and battery storage onto discrete, independently addressable inverters, coordinated through a digital control link, allows the installation to prioritise routing generation into thermal or electrochemical storage ahead of grid export, smoothing the export profile the DNO actually sees at the point of connection rather than the instantaneous generation profile at the panel.
These export-limitation and topology strategies do more than clear an individual DNO application: at scale, they reduce a property's net volumetric draw on the transmission and distribution network across the day, a dynamic examined in detail in our mathematical teardown of the Ofgem standing charge. As more G99-connected properties adopt active export limitation to avoid triggering reinforcement charges, the aggregate utilisation profile of the local distribution network shifts, and it is this same utilisation data that feeds into how Ofgem recalibrates the fixed Distribution Use of System (DUoS) and Transmission Network Use of System (TNUoS) cost allowances that underpin the standing charge every household pays.