Technical guide · G99 connections

G99 connections for UK commercial solar — 2026 guide

G99 is the engineering recommendation governing connection of distributed generation (including commercial solar above 50 kW peak) to the UK electricity distribution network. Above 50 kW, G99 application to your local DNO is mandatory. Above 200 kW, full network impact assessment is typical. Above 500 kW, network reinforcement studies often trigger. Each threshold adds time, cost, and complexity to project delivery.

When G99 applies

Connection of distributed generation to the UK electricity distribution network is governed by Engineering Recommendation G99. The thresholds:

Below 16 A per phase (3.68 kW) — G99 not required; connection under G98.
16 A to 32 A per phase (3.68-7.36 kW) — G99 Notify-Only. Inform DNO; no detailed application.
32 A to 50 kW — G99 Type Tested Connect. Standard application; typically 6-8 weeks DNO process.
50 kW to 1 MW — G99 Type A application. Standard process with assessment of network impact. 8-16 weeks typical DNO process.
1 MW+ — G99 Type B/C/D application. Detailed network impact assessment, often reinforcement requirements. 16-40 weeks typical DNO process.

Typical DNO process timeline

For a typical 200-500 kW commercial solar G99 application:

Application submission (week 0)
Submit G99 application to your local DNO via their online portal. Include site details, system specifications, and electrical infrastructure information.
Application accepted (week 1-2)
DNO confirms application is complete and assigns it to the network connections team.
Network impact assessment (week 2-8)
DNO assesses local network capacity, voltage profile, and ability to accept the proposed export. May require additional protection settings, AVR studies, or fault level assessment.
Connection offer (week 8-12)
DNO issues connection offer specifying conditions, costs, and any required network reinforcement. Offer typically valid for 90 days.
Acceptance and payment (week 12-16)
Customer accepts offer and pays connection charges. Reinforcement work scheduled if required.
Network reinforcement (if required, week 16-32)
DNO undertakes reinforcement work — transformer upgrade, cable upsizing, or substation modifications. Adds 12-20 weeks to project timeline.
Commissioning inspection (week 32+)
DNO inspector witnesses commissioning, verifies protection settings, and grants final connection consent.

Common reinforcement triggers

Network reinforcement adds material cost (£15k-£200k+) and time (3-9 months) to projects. Common triggers:

Voltage step — large generation can cause voltage rise on the network. Reinforcement may be required to maintain voltage within statutory limits.
Reverse power flow — older transformers and protection systems may not accommodate reverse power flow. Replacement or modification required.
Fault level — adding generation increases prospective fault current at downstream connections. Switchgear upgrades may be required.
Network capacity — local cables or transformers may be at capacity. Upsizing required for new generation.
Protection coordination — protection settings need to be compatible with new generation. AVR (Automatic Voltage Regulator) studies often required.

Technical FAQs

Can I install commercial solar without G99 if I don't export?

No — G99 governs connection of generation regardless of whether export is enabled. Even with export limited to zero (export-zero scheme), G99 application is required above 50 kW. Some sub-50 kW systems with export-zero settings can use G98 simplified procedures.

How much does a G99 application cost?

DNO application fees: typically £200-2,000 depending on size. Network impact assessment fees: £500-5,000 for standard 200-500 kW projects. Reinforcement charges (where required): £15k-£200k+. Total DNO costs typically 5-15% of project capex on commercial solar.

Can I avoid network reinforcement charges?

Sometimes. Options include: (a) reduce system size below the trigger threshold; (b) install export-limiting equipment to constrain peak export; (c) install battery storage to time-shift export; (d) negotiate connection at a different network point. Each has trade-offs; assess at DNO offer stage.

How does G99 differ between DNOs?

G99 is a national engineering recommendation but DNO implementation varies. UK Power Networks (London/SE), Western Power Distribution (Midlands/SW), Northern Powergrid (NE), Electricity North West (NW), Scottish Power (Scotland), SP Networks (Scotland), SSE (Scotland Highlands), and SP Manweb (Wales/NW England) each have different processes and offer letters. Most aspects are standardised but DNO-specific processes affect timeline.

What if my G99 application is rejected?

Rejection is rare. More commonly, DNO offers conditional connection requiring system modifications (export limit, additional protection, network reinforcement). If conditions are economically prohibitive, you can: (a) accept reduced system size; (b) defer until DNO upgrade plans accommodate your project; (c) negotiate alternative connection points. We work with DNO connections engineers to find acceptable solutions on most projects.

Solar export limits UK 2026

When DNO becomes the binding constraint on commercial solar sizing.

Half-hourly sizing guide

How sizing decisions interact with export consent.

Battery storage finance page

Storage as a route to manage export-limited DNO situations.

G99 connection explained for commercial solar

G99 is the Engineering Recommendation that governs the connection of larger generating units to the UK distribution network. If your commercial solar installation exceeds 50kWp output (single phase) or has any phase exceeding 50kW (three phase), you need a G99 application rather than the simpler G98 notification. G99 involves a formal assessment process by your Distribution Network Operator (DNO) and typically takes 6–13 weeks.

Understanding G99 is essential for commercial solar projects because it is often the longest item on the critical path — delays in G99 approval can hold up project commissioning even when installation is complete. Starting the G99 process early is one of the most impactful project management actions you can take.

G98 vs G99: when each applies

Connection type	System size threshold	Process	Typical timeline
G98 notification	Up to 3.68kWp per phase (16A)	Simple notification to DNO; no approval required	1–5 business days
G98 registration	Up to 50kWp single phase or 50kW per phase three phase	Submit to DNO; typically auto-approved	5–10 business days
G99 application	Above 50kWp single phase or above 50kW per phase	Formal DNO assessment; approval required before commissioning	6–13 weeks (statutory 45 working days)
G99 Stage 1 (complex)	Large or complex systems, HV connection	Detailed engineering assessment	12–20 weeks

The G99 application process step by step

Step 1: Pre-application enquiry

Submit a pre-application enquiry (PAE) to the DNO with basic system information (proposed site, system size, location, proposed connection point). The DNO provides a budgetary connection cost and identifies potential constraints. Cost: £500–1,500. Timeline: 4–10 working days.

Step 2: Formal G99 application

Submit the full G99 application pack: completed application form, single-line diagram, protection schedule, proposed inverter datasheet and G99 test certificate, site layout plan, and application fee. Most DNOs have online portals.

Step 3: DNO technical assessment

The DNO assesses technical impact of your connection on the local network (thermal capacity, voltage rise, fault level, protection coordination). Statutory period: 45 working days. In practice, many DNOs respond in 30–40 working days for straightforward applications.

Step 4: Connection offer

DNO issues a formal connection offer detailing connection point, connection cost, protection requirements, and any conditions. You have 30 days to accept or request modifications.

Step 5: Works completion

If reinforcement or new infrastructure is required, the DNO carries out the works (funded by site owner). Timelines vary: simple meter changes in days; cable reinforcement in weeks to months.

Step 6: Commissioning and testing

Before switching on the solar system, the protection relay settings are submitted to the DNO for approval. A commissioning engineer tests protection functions with DNO witness (or approved protocol). G99 commissioning certificate issued.

Common G99 issues and how to manage them

Voltage rise constraints

Rural areas with long LV cables may have voltage rise issues that limit the size of G99 system permitted. Solutions: reduce system size, install export limitation with APC, or fund DNO reinforcement.

Protection relay requirements

G99 systems above certain thresholds must include a protection relay (over/undervoltage, over/underfrequency, loss of mains). Modern solar inverters include integrated G99 protection; confirm with DNO which type is acceptable.

DNO backlog delays

Some DNOs have significant backlogs in G99 processing. Submit applications early — ideally 12–16 weeks before target commissioning date, even before finance is finalised.

UK DNO	Coverage	G99 portal	Contact
UK Power Networks (UKPN)	London, South East, East Anglia	ukpowernetworks.co.uk/connections	0800 029 4285
Western Power Distribution (WPD/National Grid)	Midlands, South West, Wales	nationalgrid.co.uk/connections	Multiple regional numbers
Northern Powergrid	Yorkshire, NE England	northernpowergrid.com/connections	0800 011 3332
Electricity North West	NW England	enwl.co.uk/connections	0800 195 4141
SP Energy Networks	Central/South Scotland, NW England	spenergynetworks.co.uk	0800 001 5400
SSEN (SSE Networks)	North Scotland, South England	ssepowerdistribution.co.uk	0800 783 8866

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