DNO G99 connection process for UK commercial solar
Every UK commercial solar PV system above 16A per phase requires a G99 application to your Distribution Network Operator (DNO) before connection. The process typically takes 5-11 weeks, sometimes longer for sites requiring capacity studies. This guide covers the application, timeline, charges and common rejection causes.
What G99 covers and why
G99 (Engineering Recommendation G99) governs the connection of generating plant to the UK distribution network. Published by the Energy Networks Association, it covers technical standards for protection settings, fault levels, earthing, witness testing and commissioning. For commercial solar specifically, G99 ensures:
- Network operators can isolate generation during planned outages or faults - Generation doesn't degrade voltage or power quality on the local network - Protection systems work correctly during grid faults - Local network can absorb the exported generation without thermal overloading
Without G99 approval, you cannot commission a system above 16A per phase. Below that threshold, G98 'connect and notify' applies — simpler process but only suits very small systems (typically under 11kWp on three-phase, under 3.7kWp on single-phase).
Application timeline
Typical G99 application stages and durations:
- Pre-application enquiry (1-2 weeks): Initial check with the DNO confirming feasibility and indicative capacity. Often free. - Full application (2-3 weeks): Submit single-line diagram, inverter datasheets, panel specifications, earthing arrangement, protection settings. - Technical review (4-8 weeks): DNO load flow studies for systems above 17kWp; full capacity studies for systems above 250kWp. - Connection offer (1-2 weeks): DNO issues offer defining export limit, connection charges, technical conditions. - Acceptance and payment: You accept the offer and pay connection charges. - Commissioning and witness testing (1-2 weeks): DNO attends for systems above 50kWp; remote sign-off for smaller systems.
Total: 8-16 weeks typical. Rural sites with constrained capacity can extend to 26 weeks. Sites requiring network reinforcement (new transformer, additional feeder cable) can extend further to 6-12 months.
Start your G99 application no later than the day you sign your installer contract.
Export limits — the curtailment trap
The single biggest G99 gotcha is the export limit. The DNO assesses local network capacity and may impose a limit lower than your system's nameplate. Common examples:
- 200kWp on rural single-phase upgrade: 50kW export limit (75% curtailment in low-load periods) - 500kWp on industrial estate: 250kW export limit (50% curtailment) - 1MWp on big-box warehouse: 500kW or full export — varies by region
When self-consumption is high, export limits don't bite. When self-consumption is low (warehouses, weekends, school holidays), the inverter curtails generation rather than exporting beyond the limit. That generation is lost — permanent revenue forgone.
Mitigations:
1. Right-size the system for the export limit plus typical self-consumption 2. Battery storage to time-shift excess generation to higher-load hours 3. Negotiate higher limit if DNO has spare capacity (sometimes available for additional connection charge) 4. Accept the limit and plan the financial model around it
For projects above 250kWp, request a Connection Boundary Charge offer to understand the cost of higher export capacity.
Connection charges 2026
G99 connection charges vary by DNO, by project size, by network area. Indicative ranges:
- Standard application fee: £500-£2,000 - Witness testing fee (systems above 50kWp): £500-£1,500 - Capacity study fee (systems above 250kWp): £2,000-£8,000 - Connection assets contribution (where DNO provides additional equipment): £0 to £50,000+ - Network reinforcement (where required): £15,000 to £500,000+ - Commissioning attendance fee: £500-£2,000
Budget £3,000-£8,000 for DNO costs on a typical 100-500kWp commercial project. Network reinforcement is the wild-card — get an indicative figure during the pre-application phase before committing to system size.
For sites with binding network constraints, sometimes funding reinforcement is economic; sometimes downsizing the system is better.
Common application failure modes
From DNO reports, three patterns cause ~80% of G99 application problems:
1. Incomplete documentation. Missing single-line diagrams, panel/inverter datasheets, protection relay settings, or earthing arrangement details. Adds 2-4 weeks to timeline.
2. Network capacity rejection. DNO determines insufficient local capacity. Options: accept reduced export limit, fund network reinforcement, downsize system. Negotiate before accepting.
3. Type test certification gaps. Inverters must hold valid G99-Type-Test certification. Older inverters or grey-market imports sometimes don't. Confirm before specifying — major brands (SolarEdge, Huawei, SMA, Sungrow) certify all UK-market products.
Less common but expensive failure modes: - Protection coordination conflicts with existing site equipment - Earthing system conflicts with shared distribution transformer - Site requires interleaving with HV switchgear upgrades - Network code changes mid-application (rare; UK is stable)
The 6 main UK DNOs
UK distribution network operators cover defined geographic regions:
- UK Power Networks (UKPN): London, South East, East England - Northern Powergrid: North East, Yorkshire - SP Energy Networks (SPEN): Central Scotland, South Wales - Western Power Distribution (WPD): Wales, Midlands, South West - Electricity North West (ENW): North West - Scottish & Southern Electricity Networks (SSEN): Scotland (north), Central Southern England
Plus various Independent Distribution Network Operators (IDNOs) on large industrial estates and new-build developments. The IDNOs are licensed under separate Engineering Recommendation but follow the same G99 principles.
DNO contact details and connection portals are listed on the Energy Networks Association website. Use the DNO's online portal for application submission where available — faster than email/post.
DNO G99 connection process for UK commercial solar · FAQs
How long does a G99 application take?
Typical 8-16 weeks end-to-end for commercial systems up to 250kWp. Larger systems requiring capacity studies extend to 16-26 weeks. Sites needing network reinforcement: 6-12 months.
Can I appeal a low export limit?
Yes — request a Connection Boundary Charge offer for the higher capacity you need. The DNO will quote network reinforcement costs. Sometimes economic, sometimes not.
Who submits the G99 application?
Your installer typically submits on your behalf. Confirm in writing that they hold design responsibility and can handle DNO queries. Some installers sub-contract this to specialist DNO consultants.
Do I pay for witness testing?
Yes for systems above 50kWp. Typical fee £500-£1,500. The DNO sends an engineer to attend commissioning and confirm protection settings work correctly.
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