Engineering Recommendation G99 sets the technical and connection-process requirements for generating equipment that operates in parallel with a licensed distribution network in Great Britain and is not within, or does not comply with, G98. It is part of the Distribution Code rather than a product certificate that can be added to a site after installation.
The current source reviewed for this guide is G99 Issue 1 Amendment 10 (2024). Connection documents change, so confirm the latest Distribution Code publication and the requirements of the relevant distribution network operator (DNO) before designing or modifying a project.
G98 or G99: start with the connection scope
G98 covers Fully Type Tested micro-generators rated up to and including 16 A per phase that connect in parallel with a public low-voltage distribution network. G99 covers power-generating modules outside that scope, as well as equipment that is not compliant with the G98 requirements.
That distinction is about more than the nameplate power of one inverter. A connection assessment can depend on the generating units, how they are grouped into modules, the aggregate facility, the connection point, existing generation or storage, and whether an export-limitation scheme is proposed.
Do not choose a process from a headline kW figure alone. The installer or appointed technical representative should establish the project configuration with the DNO, including additions to an existing site and equipment that has been substantially modified.
G99 Types A, B, C and D
G99 classifies a power-generating module by Registered Capacity and connection voltage. The current document defines the four types as follows:
| G99 type | Registered Capacity and connection point |
|---|---|
| Type A | At least 0.8 kW and below 1 MW, with a connection point below 110 kV |
| Type B | At least 1 MW and below 10 MW, with a connection point below 110 kV |
| Type C | At least 10 MW and below 50 MW, with a connection point below 110 kV |
| Type D | At least 50 MW, or a connection point at or above 110 kV |
The 0.8 kW lower boundary does not move a compliant micro-generator out of G98. G99 itself excludes G98-compliant modules within the G98 scope. It also distinguishes a power-generating module from the wider power-generating facility, which can contain several modules of different types.
Capacity can trigger obligations outside G99. The recommendation notes different thresholds for Grid Code treatment and power-station classification across Great Britain. Those rules, generation licensing, market participation and the connection agreement must be assessed separately rather than inferred from the table above.
What a G99 connection assessment covers
The work becomes more extensive as project size and system impact increase. Depending on the module type and site, the evidence can include:
- connection applications and DNO network studies;
- the proposed connection design and interface protection;
- type-test evidence or project-specific compliance evidence;
- voltage, frequency and reactive-power behaviour;
- fault-ride-through and fast-fault-current capability for the types to which those requirements apply;
- simulation studies for larger modules;
- commissioning tests and installation documents; and
- confirmation that the commissioned settings match the accepted design.
The connection offer and connection agreement can add site-specific requirements. Passing a factory type test does not by itself demonstrate that the complete installation, protection settings, communications and commissioned plant meet those requirements.
How storage and vehicle-to-grid fit
G99 includes electricity storage in its definition of a power-generating module when the equipment can export in parallel with the distribution network. It also covers vehicle-to-grid electric vehicles in that exporting role. The ENA describes G98 and G99 as part of the storage connection process, and G99 states that storage commissioned on or after 1 September 2022 must comply with the recommendation in full.
Import behaviour, export limits and the wider installation can introduce additional requirements. An export-limitation scheme may need to meet G100, but installing such a scheme does not remove the need to agree the connection route with the DNO.
What monitoring evidence G99 actually asks for
G99 is not a general specification for an energy dashboard. Its formal evidence is tied to connection compliance, commissioning and the connection agreement.
For Type C and Type D modules, Annex C.6 defines a functional specification for dynamic system monitoring, fault recording and power-quality monitoring. The exact requirement, particularly whether power-quality monitoring is needed, is agreed bilaterally and recorded in the connection agreement. The annex sets requirements for matters such as recorded quantities, triggering, timing, storage and data access.
Type A and Type B projects use their applicable compliance documents, commissioning checks and installation confirmations. A separate operational monitoring system can help an owner retain evidence and diagnose performance, but its readings are not automatically the measurements, tests or records required by the DNO.
A practical site measurement plan
For operations, asset management and platform integration, a useful measurement plan often separates four evidence boundaries:
- Point of connection: import, export, voltage, current, active power, reactive power, power factor and frequency at the agreed boundary.
- Generating module output: the electrical output and availability of each relevant inverter, generator or storage converter.
- Control and protection state: accepted setpoints, run state, limiting state, trips and acknowledgements from the plant controller or protection system.
- Context: local load, storage state of charge, irradiance or other process conditions needed to explain why export changed.
Those signals should use a common time reference, clear units and retained raw values. The required sampling interval, timestamp performance, event capture and retention period should come from the connection agreement, market service and operating objective, not from a generic IoT default.
This operational plan is a design recommendation, not a substitute for the G99 compliance plan. Measurements used for settlement, protection, formal compliance testing or DNO fault investigation may need specified instruments, classes, calibration and access arrangements.
Where EpiSensor can fit
EpiSensor can provide an independent site-monitoring layer around distributed generation: electricity measurement at selected circuits or boundaries, acquisition of supported Modbus and analogue signals, local logging and processing in Edge, and onward integration with an owner, aggregator or energy platform.
That layer can help teams compare inverter telemetry with an independent electrical measurement, retain operational context and investigate a curtailment or availability event. It does not provide G99 approval, certify an inverter, replace interface protection, perform the DNO's network study, or turn ordinary monitoring data into compliant settlement or fault-recording evidence.
For a new project, use the solar PV monitoring guide to plan the independent measurement points. If the equipment and signals are still uncertain, start with the System Builder and treat the result as a monitoring architecture, not a connection design.
Before applying or commissioning
- Identify the DNO and confirm the current application route.
- Record every generating unit, storage device, inverter and existing source at the site.
- Establish the module and facility structure before assigning a G99 type.
- Confirm type-test references and the exact firmware or settings to which they apply.
- Agree interface protection, export limitation and site-specific connection conditions.
- List every required study, form, test, witness point and commissioning record.
- Agree the monitoring and data-access requirements in the connection agreement.
- Keep operational monitoring, certified metering, protection and formal compliance evidence as explicit, separate responsibilities.
G99 is therefore best treated as a connection lifecycle: classify the project correctly, agree the design with the DNO, demonstrate the applicable technical behaviour, commission the accepted installation and retain the evidence. Site monitoring becomes valuable when it is designed around those boundaries and kept honest about what it can prove.