Connecting a domestic solar photovoltaic array or domestic energy storage system to the British electricity network requires strict compliance with the Energy Networks Association (ENA) technical engineering recommendations. For most UK homeowners and electricians, navigating grid connection permissions centres entirely on understanding G98 vs G99 DNO approval UK regulations.
Failing to submit the correct notification or prior permission form to your regional Distribution Network Operator (DNO)—such as UK Power Networks, National Grid Electricity Distribution, Scottish and Southern Energy Networks (SSEN), or SP Energy Networks—can result in your export being officially revoked or your Feed-in/Smart Export Guarantee (SEG) payments being delayed indefinitely.
What Is a DNO and Why Do They Regulate Grid Export?
While energy retailers (such as Octopus Energy or British Gas) bill you for energy consumption, regional DNOs own and maintain the physical cables, substations, and transformers delivering power to your property. When thousands of households inject unmonitored solar and battery power back into the 230V single-phase grid simultaneously, local network voltages can spike above statutory limits (230V +10% / -6%), overloading regional substations.
Engineering Recommendation G98: Connect and Notify
Engineering Recommendation G98 covers fully type-tested, micro-generation equipment designed for simple domestic installation. The defining principle of a G98 installation is “Connect and Notify”:
- Single-Phase Capacity Ceiling: The total aggregate AC generating capacity cannot exceed 16A per phase, which equates to exactly 3.68kW at nominal 230V single-phase supply.
- Three-Phase Properties: The aggregate limit is 16A per phase, allowing up to 11.04kW across all three phases.
- Process: The certified installer commissions the system and simply notifies the regional DNO within 28 days of commissioning using standard ENA notification documentation. No advance permission or network survey fee is required.
Engineering Recommendation G99: Prior Authorisation Required
Any generating unit exceeding 16A per phase (over 3.68kW on a single-phase supply) automatically falls under Engineering Recommendation G99. Unlike G98, G99 follows an “Apply and Wait” protocol.
You cannot legally connect and energise a G99 installation until the regional DNO assesses the local grid network capacity and grants formal, written approval. Depending on network density, receiving G99 permission typically takes between 4 to 10 weeks.
The Battery Storage Catch: Combined Inverter Inadvertent Export
A frequent compliance trap arises when homeowners retrofit an AC-coupled battery storage system to an existing solar array:
- If you already have a 3.68kW solar inverter running under G98, and you add a separate 3.6kW battery storage inverter, your home’s total combined potential generation capacity is now 7.28kW AC.
- Because 7.28kW exceeds the 3.68kW single-phase threshold, the combined system no longer complies with G98 and requires a full G99 application.
- The G99 Fast-Track Solution: Many modern installations utilise hybrid inverters or dedicated G100-compliant export limitation devices configured to throttle maximum back-feed to 3.68kW. Under the ENA Fast-Track G99 A1-1 process, applications for type-tested systems with export limitation can be processed significantly faster.
Comparison Matrix: G98 vs G99
| Requirement | G98 Standard | G99 Standard |
|---|---|---|
| Max Single-Phase Export | 3.68 kW (16A) | Above 3.68 kW |
| DNO Permission Timing | Retrospective (Notify within 28 days) | Prior Written Approval Mandatory |
| Typical DNO Turnaround | Immediate / Automated | 4 to 10 weeks |
| Application Fees | Free | Usually free for domestic; complex commercial may incur fees |
| Common Equipment | Small standard 3kW–3.6kW string inverters | 5kW–8kW hybrid inverters, dual-inverter setups |
Steps to Secure Your DNO Approval Without Delays
- Locate Your MPAN and DNO: Identify your regional network operator via your electricity bill’s Meter Point Administration Number (MPAN).
- Check Main Cut-Out Fuse Rating: Before submitting a large G99 application, verify whether your property has a 60A, 80A, or 100A main service cut-out fuse. Older 60A supplies may require a free fuse upgrade from the DNO.
- Ensure G100 Export Limitation Compliance: If installing an inverter rated at 5kW or 6kW while seeking rapid approval, instruct your installer to specify integrated G100-certified fail-safe export throttling to 16A (3.68kW).
- Obtain the DNO Acceptance Letter: Retain the official DNO approval letter and equipment commissioning sheets alongside your MCS certificate, as both are mandatory when registering for smart export tariffs like Octopus Outgoing or Flux.