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:

  1. Application submission (week 0)

    Submit G99 application to your local DNO via their online portal. Include site details, system specifications, and electrical infrastructure information.

  2. Application accepted (week 1-2)

    DNO confirms application is complete and assigns it to the network connections team.

  3. 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.

  4. Connection offer (week 8-12)

    DNO issues connection offer specifying conditions, costs, and any required network reinforcement. Offer typically valid for 90 days.

  5. Acceptance and payment (week 12-16)

    Customer accepts offer and pays connection charges. Reinforcement work scheduled if required.

  6. 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.

  7. 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.

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