Blog · Published 2026-05-12

G99 application guide for commercial solar in 2026

Every commercial solar PV system above 16A per phase needs a G99 application to your Distribution Network Operator (DNO) before it can be connected and commissioned. This is the single most-frequently underestimated lead time on UK commercial solar projects — typically 5-11 weeks, sometimes longer for sites requiring capacity studies. This guide walks through the process and the gotchas that cost commercial solar projects months.

What is G99 and why does it matter

G99 (Engineering Recommendation G99) is the UK technical specification governing how generating plant — solar PV, wind, CHP — connects to the public electricity distribution network. Published by the Energy Networks Association, it replaced the older G83 standard in 2018. G99 covers everything from connection terms to protection settings to commissioning testing.

For commercial solar specifically, G99 applies to any system above 16A per phase. For typical UK commercial three-phase supplies, this means anything above approximately 11kWp triggers G99. Below that threshold, the simpler G98 'connect and notify' regime applies. Above 50kWp, additional witness testing and detailed engineering review apply.

The G99 application is to your local Distribution Network Operator (DNO). UK has 6 main DNO regions: UK Power Networks (East + South-East), Northern Powergrid (North-East + Yorkshire), SP Energy Networks (Central Scotland + South Wales), Western Power Distribution (Wales + Midlands + South-West), Electricity North West (North-West), and SSEN (North Scotland + South-Central). Plus various Independent DNOs (IDNOs) on large industrial estates.

Application timeline

Typical end-to-end G99 timeline for a commercial solar project:

- Week 1-2: Pre-application enquiry to DNO confirming feasibility and indicative capacity. - Week 3-4: Full application submitted with technical drawings, single-line diagrams, panel and inverter datasheets. - Week 5-11: DNO technical review. Includes load flow studies for systems above 17kWp; full capacity studies for systems above 250kWp. - Week 11-12: Connection offer issued — defines export limit, connection charges, technical conditions. - Week 13: Acceptance and payment of connection charges. - Week 14+: Commissioning and DNO witness testing for systems above 50kWp.

Rural sites with constrained network capacity can extend this to 16-26 weeks. Sites requiring network reinforcement (transformer upgrades, new feeder cables) can extend further.

Start the G99 application no later than the day you sign the installer contract. Earlier is better.

Export limits and 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 generation. Common examples:

- 200kWp system on rural single-phase upgrade: 50kW export limit (75% curtailment in low-load periods) - 500kWp system on industrial estate: 250kW export limit (50% curtailment) - 1MWp system on big-box warehouse: 500kW or full export — varies by region

When self-consumption is high, export limits don't hurt. When self-consumption is low — warehouses, weekends, school holidays — the inverter curtails generation rather than exporting beyond the limit. That lost generation is permanent revenue forgone.

Mitigations: (a) right-size the system for the export limit plus self-consumption, (b) install battery storage to time-shift excess generation to higher-load hours, (c) negotiate higher limits where DNO has spare capacity (sometimes available for an additional connection charge).

Common application failure modes

Three repeat failure modes account for ~80% of G99 application problems:

1. Incomplete documentation. Missing single-line diagrams, panel/inverter datasheets, protection relay settings, or earthing arrangement details. Add 2-4 weeks to the timeline.

2. Network capacity rejection. DNO determines local capacity insufficient. Options: accept reduced export limit, fund network reinforcement (£15k-£500k typically), or downsize the system. Negotiate.

3. Type test certification gaps. Inverters must hold valid G99-Type-Test certification. Older inverters or grey-market imports may not. Confirm before specifying.

Less common but expensive failure modes: (a) protection coordination conflicts with existing site equipment, (b) earthing system conflicts with shared distribution transformer, (c) site requires interleaving with HV switchgear upgrades, (d) network code changes mid-application (very rare — UK is stable).

Connection charges and ancillary costs

G99 connection charges vary widely. Indicative ranges (2026):

- Standard application fee: £500-£2,000 - Witness testing fee (above 50kWp): £500-£1,500 - Capacity study fee (above 250kWp): £2,000-£8,000 - Network reinforcement (where required): £15,000-£500,000 - Connection commissioning attendance: £500-£2,000

Budget £3,000-£8,000 for DNO costs on a typical 100-500kWp project. Network reinforcement is the wild-card — get an indicative figure during pre-application before committing to the system size.

FAQs

FAQs on this topic

Do I need G99 for my commercial solar?

Yes if your system exceeds 16A per phase. For typical UK commercial three-phase supplies, this is approximately 11kWp. Above that threshold, G99 application is mandatory.

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 the reinforcement is economic; sometimes not.

Who submits the G99 application?

Your installer typically submits on your behalf. Confirm in writing they hold the design responsibility and can handle DNO queries. Some installers sub-contract this to specialist DNO consultants.

What happens after commissioning?

DNO issues a Final Connection Offer once witness testing passes. You're then formally connected and can claim SEG. Most modern smart-metered sites need no further action.