A G99 application is required for any solar PV system that exports more than 16 amps per phase to the UK distribution network. Unlike G98 (fit-and-inform), G99 requires formal DNO approval before installation begins — not after.
Getting the application right first time avoids the 45-day clock restarting. This guide walks through the complete process from initial design to commissioning sign-off.
Before You Apply: Confirming G99 is Required
Before preparing any documents, confirm that G99 actually applies to the project.
Step 1: Calculate export current per phase
For single-phase: divide inverter AC output (W) by 230V. For three-phase: divide inverter AC output (W) by 690 (3 × 230).
A 5 kW single-phase inverter exports 5000 ÷ 230 = 21.7A — G99 required. A 10 kW three-phase inverter exports 10,000 ÷ 690 = 14.5A per phase — G98 applies.
Step 2: Check combined capacity
Add the export current of all existing generating plant at the same connection point. If the combined total (existing plus proposed) exceeds 16A per phase, G99 applies even if the new system alone would qualify for G98.
Step 3: Check DNO-specific conditions
Some DNOs have imposed additional connection conditions at specific network locations — for example, requiring export limitation on all new generation above a certain capacity in a particular substation area. Check with the DNO or their online portal before assuming standard G99 terms apply.
If G98 applies instead, see the G98 vs G99 guide for the notification process.
Stage 1: Design Submission
Stage 1 is the formal design review. No installation work can begin until the DNO issues a Stage 1 acceptance (or deemed acceptance applies).
Document Checklist for Stage 1
Completed G99 Application Form
Download the current form from the relevant DNO’s website. Fill in: installation address, site owner details, installer details (name, company, MCS certificate number if applicable), system capacity (kWp DC and kW AC), inverter make and model, number of phases, proposed connection date, and whether export limitation is used. Each DNO has its own form — do not mix forms between DNOs.
Single-Line Diagram
The single-line diagram (SLD) is the most important document in the G99 application. It must show: the PV array (capacity in kWp, string count), DC isolator(s), inverter(s) with make, model, and rated AC output, AC isolator, protection relay identification (integrated or external), generation meter (if required), import/export meter, export limiting device (if applicable), and the connection to the DNO network including the service head. Use standard IEC or BS EN electrical symbols. Label all voltage levels, conductor sizes, and fuse/breaker ratings.
Inverter Data Sheet and ENA Type-Test Certificate
Supply the inverter manufacturer’s data sheet showing AC output, protection relay specification, and the ENA type-test certification reference number. The inverter must hold a valid ENA type-test certificate for the connection type (single or three-phase) and voltage level. Check the ENA’s Type Test Register to confirm the certificate is current before submitting. An expired or invalid type-test certificate is one of the most common reasons for G99 rejection.
Protection Settings Schedule
The Protection Settings Schedule is a table of all protection relay parameters that will be set in the inverter. Standard G99 default settings are specified in the Engineering Recommendation. The schedule must show the setting value, the permitted range from G99, and confirmation that the inverter’s type-test certificate covers the setting. Some DNOs provide their own Protection Settings Schedule template — use this if available.
Export Limitation Details (if applicable)
If the system uses export limitation — for example, a zero-export or fixed export cap arrangement — provide details of the export limiting device: make, model, type-test certificate reference, and how it interfaces with the inverter. Some DNOs require export limitation as a condition of connection for certain network locations; others accept it as an alternative to network reinforcement. State the proposed maximum export level in kW and kVA.
What the DNO Assesses in Stage 1
The DNO’s Stage 1 review checks three things: that the proposed generation will not exceed the available network capacity at the connection point; that the protection relay settings will keep the system within G99 compliance; and that the application documents are complete and consistent. The DNO may carry out a load flow study for larger systems to assess voltage impact on neighbouring customers.
The 45-Working-Day Review Period
From the date the DNO confirms receipt of a complete application, the DNO has 45 working days to:
- Issue a connection offer (Stage 1 acceptance)
- Reject the application with reasons
- Request additional information (which restarts the 45-day clock when supplied)
If no response is received within 45 working days and no information request has been issued, deemed acceptance applies. The installation proceeds as if a formal Stage 1 acceptance had been issued, on the terms of the original application.
Tracking the Clock
Keep the following records:
| Record | Why It Matters |
|---|---|
| Submission date | Establishes when the 45-day clock starts |
| DNO acknowledgement email or portal receipt | Confirms the DNO received a complete application |
| Any DNO information requests and your replies | Tracks any clock restarts |
| Deemed acceptance date (day 46+) | Your evidence if the DNO disputes the status |
Deemed Acceptance Requires a Complete Application
The 45-day deemed acceptance rule only applies if the original submission was a complete application. If the DNO considers the application incomplete — because a required document was missing or a form field was blank — the clock does not start. Submit everything in one package and confirm receipt before relying on the 45-day timeline.
Common Rejection Reasons
Understanding why G99 applications fail helps avoid them. The most common rejection reasons:
| Rejection Reason | How to Avoid It |
|---|---|
| Incomplete single-line diagram | Use the DNO’s own SLD checklist; include all protection, metering, and connection points |
| Protection Settings Schedule blank or out of range | Use the inverter’s type-test certificate values; confirm they fall within G99 permitted ranges |
| Inverter not ENA type-tested | Check the ENA Type Test Register before specifying the inverter |
| Type-test certificate expired | Confirm certificate validity date before submission |
| Export limiting device not specified where required | Check DNO connection conditions for the specific network area |
| Combined capacity calculation missing | Always declare all existing generation at the connection point |
| Wrong application form used | Download the form from the correct DNO’s current website |
Stage 2: Commissioning
After receiving Stage 1 acceptance (or after deemed acceptance applies), installation can begin. Stage 2 covers commissioning and the post-commissioning notification.
The Commissioning Process
Program Protection Relay Settings
Configure the inverter with exactly the protection relay settings agreed in the Stage 1 approval. Use the Protection Settings Schedule from the Stage 1 submission as the reference. Do not change any settings from those agreed without first obtaining DNO consent — doing so constitutes a material change requiring a new Stage 1 application.
Carry Out the Protection Relay Verification Test
Test each protection relay function to confirm it operates at the correct set point. For voltage and frequency protection, this typically involves using the inverter’s built-in test mode or a dedicated relay test set to simulate out-of-range conditions. Record the test results on the commissioning test sheet. See the G99 Commissioning Test guide for the full test procedure and record format.
Complete the Electrical Installation Certificate
Issue the BS 7671 Electrical Installation Certificate for the installation. The certificate must reference the G99 application reference number and confirm that the electrical installation complies with BS 7671, 18th Edition including Amendment 2, and with Part 7 Section 712 (solar PV systems).
Submit Stage 2 Post-Commissioning Notification
Submit the Stage 2 notification to the DNO confirming the system has been commissioned as approved. Include: the commissioning test results, the Electrical Installation Certificate reference, the actual commissioning date, the serial numbers of all inverters installed, and confirmation that the protection settings match those agreed in Stage 1. Some DNOs require a copy of the commissioning test sheet to be attached.
Witnessed Tests
For larger G99 systems, the DNO may require the commissioning test to be witnessed by a DNO representative or an independent engineer approved by the DNO. For residential and most small commercial G99 systems (typically below 50 kW), a witnessed test is not normally required. Confirm with the DNO whether a witnessed test is required before scheduling commissioning.
Per-DNO Application Notes
UK Power Networks (UKPN)
UKPN operates an online portal for both G98 notifications and G99 applications. G99 applications are submitted through the UKPN Connections portal. UKPN provides a pre-application checking service for larger projects. Stage 1 applications for systems below 50 kW are processed through a dedicated small-scale generation team. See the UKPN guide for the current portal URL and form requirements.
NGED (National Grid Electricity Distribution)
NGED — which absorbed Western Power Distribution in 2023 — operates the Connected Networks portal for G99 applications across the Midlands, South West, and South Wales. NGED has published a specific small-scale generation guide that walks through the G99 application requirements for systems below 1 MW. See the NGED guide for details.
Northern Powergrid
Northern Powergrid covers North East England and Yorkshire. G99 applications are submitted through a combination of the Northern Powergrid connections portal and, for some application types, email to the distributed generation team. Northern Powergrid publishes a Distributed Generation Policy document that sets out additional connection requirements for its network. See the Northern Powergrid guide.
Electricity North West (ENW)
ENW covers North West England. G99 applications are submitted through the ENW connections portal. ENW operates a streamlined process for solar PV systems below a defined capacity threshold in areas with available network headroom. See the ENW guide.
SP Energy Networks (SPEN)
SPEN covers Scotland and North Wales, operating two licensed networks: SP Distribution (Scotland south of the Central Belt and North Wales) and SP Manweb (North Wales). G99 applications are submitted through the SPEN connections portal. Scotland has some additional grid capacity constraints in rural areas — SPEN may impose export limitation as a condition of connection in some network areas. See the SPEN guide.
SSE Networks
SSE Networks covers the Scottish Highlands and Islands and parts of Southern England. The Scottish Highlands network has significant renewable generation and some constraint areas where export limitation or staged connection conditions are imposed. G99 applications are submitted through the SSE Networks connections portal. See the SSE Networks guide.
One Application Package, No Missing Documents
The most effective way to avoid the 45-day clock being extended is to submit a complete, well-formatted application in a single package. Include a cover sheet listing every document included and its page count. When the DNO’s reviewer opens the submission, a clear cover sheet signals a professional submission and reduces the chance of a follow-up request for a document that was actually included but not clearly identified.
Using Solar Design Software for G99 Applications
Solar software built for the UK market can significantly reduce the time spent preparing G99 application documents.
A purpose-built solar design software workflow for G99 should output:
- A single-line diagram in the format DNOs require, auto-populated from the project design
- A Protection Settings Schedule with all relay parameters drawn from the inverter’s ENA type-test certificate data
- The system summary table for the G99 application form (capacity, phases, connection details)
- Inverter data sheet packaged with the correct ENA certification reference
Producing these documents manually from a separate design file and equipment data sheets is error-prone. Inconsistencies between the SLD and the application form — for example, a mismatched kW figure — are among the most common causes of DNO information requests. Solar proposals software that links design data directly to compliance documentation eliminates most of these inconsistencies.
See the Solar Design Software UK comparison for a review of tools that support G99 documentation.
Frequently Asked Questions
What documents do I need for a G99 application?
A complete G99 Stage 1 application requires: the DNO’s application form, a single-line diagram, the inverter data sheet with ENA type-test certificate reference, a completed Protection Settings Schedule, and export limitation details if the system uses export limitation. Submitting all documents in one package avoids the 45-day clock being delayed by information requests.
How long does a G99 application take?
The DNO has 45 working days from receipt of a complete application. Most residential and small commercial G99 applications are decided in 4–8 weeks. The clock restarts if the DNO requests additional information.
What is deemed acceptance?
If the DNO does not respond within 45 working days of receiving a complete application, the connection is treated as approved on the terms of the original application. Keep clear records of the submission date and the DNO’s acknowledgement — these documents establish that deemed acceptance applies if disputed.
Can I start installation before G99 approval?
No. G99 requires formal DNO pre-approval before installation begins. Installing or energising a system without G99 acceptance is a breach of distribution licence conditions. The only exception is preparatory works that do not involve connecting generation equipment to the grid.
What are the most common reasons a G99 application is rejected?
The most common reasons are: incomplete or incorrectly formatted single-line diagram, Protection Settings Schedule blank or using out-of-range values, inverter not holding a valid ENA type-test certificate, and failure to declare existing generation at the connection point. All of these are avoidable with thorough preparation before submission.
