What is the Difference Between G98 and G99 Applications for Solar?
G98 and G99 are the two engineering recommendations that govern how solar PV systems connect to the UK electricity grid. Getting this wrong – installing under G98 when G99 was required – is a compliance issue that can require system disconnection and rework. Understanding which applies is a core competency for any solar installer.
The Short Version
| G98 | G99 | |
|---|---|---|
| Export capacity threshold | ≤3.68kW single-phase | >3.68kW single-phase |
| Timing | Notify within 28 days after install | Apply before install, get approval |
| Process | Notification only | Application + technical assessment |
| Wait time | None | 4-12+ weeks |
| Suitable for | Most residential | Larger domestic, commercial |
Engineering Recommendation G98
G98 (Engineering Recommendation G98, published by Energy Networks Association) covers connection and parallel operation of generating units up to and including 16A per phase (3.68kW for single-phase).
Who it applies to: The vast majority of residential solar installations. A typical 3.5kWp domestic system with a 3.68kW inverter falls under G98.
The G98 process: 1. Complete and commission the installation 2. Submit a G98 notification to the local DNO within 28 days 3. No prior approval is required – it’s a notification, not an application 4. The DNO registers the connection
G98 notifications are submitted online via each DNO’s portal (see individual DNO websites). You’ll need the installation address, MPAN, inverter details, and MCS certificate number.
Engineering Recommendation G99
G99 (Engineering Recommendation G99) covers connection and parallel operation of generating units above the G98 threshold, and all generating units connected to high voltage networks.
Who it applies to: – Single-phase systems with inverter export capacity above 3.68kW – Three-phase systems above 11.04kW – Any installation where the DNO determines a technical study is needed (even below these thresholds)
The G99 process: 1. Submit a G99 application to the DNO before commencing installation 2. DNO carries out a technical assessment (may include power flow studies for larger systems) 3. DNO issues connection approval with any technical requirements 4. Install the system per the approved design 5. Submit commissioning notification
Timeline: DNOs have statutory timescales – Stage 1 response within 10 business days for most applications – but in practice timelines of 4-12 weeks are common. For larger or more complex connections, allow longer.
The 3.68kW Threshold Explained
The 3.68kW threshold relates to the inverter’s maximum export capacity, not the panel capacity.
- A 4kWp panel array + 3.68kW inverter (export-limited) G98 (single phase)
- A 4kWp panel array + 4kW inverter G99 (single phase)
This is why many residential systems are designed with 3.68kW inverters – it keeps them within G98 and avoids the G99 approval process entirely.
Important for battery systems: if a battery system can export to the grid (e.g. a battery charged from the grid that can also export), the total maximum export capacity includes the battery inverter. This can push a system from G98 to G99 territory. Check carefully.
Practical Implications for Solar Installers
At the survey stage: – Determine the proposed system size – Check whether the inverter export capacity is above or below 3.68kW (single phase) – If G99 applies, advise the customer of the timeline implications before confirming a start date
At the design stage: – Consider whether export-limiting the inverter to 3.68kW is appropriate for the customer’s needs – If the customer wants maximum export capability from a larger system, factor in G99 timeline
At installation: – If G98: complete install, submit notification within 28 days – If G99: ensure DNO approval is received before starting electrical work
Documentation: – Keep a copy of every G98 notification confirmation and every G99 approval letter – Include these in your MCS installation records – Provide copies to the customer as part of handover documentation
Common Mistakes
Assuming G98 for all residential jobs – a 4kWp system with a 4kW inverter is G99. Check the inverter spec, not just the panel capacity.
Missing the G98 28-day deadline – a small but real compliance issue. Set a calendar reminder for every installation.
Not factoring G99 timescales into quotes – quoting a start date before G99 approval has been received creates expectation management problems. Be explicit: “This system requires G99 DNO approval. Timeline to installation is approximately [X] weeks from application.”
Wrong MPAN on notifications – the MPAN links the grid notification to the specific supply point. An incorrect MPAN can delay processing and requires correction.
Conclusion
The trades industry rewards those who combine excellent work with professional business practices. The guidance above covers the practical fundamentals – applying it consistently is what separates the tradespeople who stay busy and profitable from those who struggle with feast-and-famine cycles. For further guidance, visit Energy Networks Association: G98/G99.
Frequently Asked Questions
Can I install a solar system before the G99 application is approved?
No. For G99 systems, installation cannot proceed before DNO approval is issued. Installing without approval is a breach of the electricity connection requirements and can result in required disconnection.What if I install a G99-applicable system under G98 by mistake?
This is a compliance issue. The DNO can require disconnection until a proper G99 approval is obtained. The system would need to be retested and potentially modified. The impact on the customer’s SEG eligibility and insurance could also be significant. Always check.How do I submit a G98 notification?
Each DNO has its own online portal. For example, UK Power Networks G98 portal, National Grid ED Connected Networks. You’ll need the installation address, MPAN, inverter details, commissioned date, and your MCS certificate number.Does G99 cost anything to apply for?
For most smaller domestic connections, the G99 technical assessment is free. Larger or more complex applications may incur a connection study fee. Check with your local DNO.Related reading:
