What are the requirements for a G99 grid connection application?

Most businesses planning a commercial solar installation never think about the G99 process until it delays their project.

That is the moment someone explains it is not just a form. It is a formal application to your Distribution Network Operator (DNO) that needs to be approved before a single inverter gets installed. Submit it late, or submit it with gaps, and weeks very quickly turn into months.

This article sets out exactly what the G99 process is, who needs it, what you are required to submit, and what actually determines how long the process takes. If you are planning a commercial solar installation in the UK, it is worth understanding this before you finalise your project timeline.

What is G99?

G99, formally known as Engineering Recommendation EREC G99 and published by the Energy Networks Association (ENA), is the technical standard that governs how generating equipment (like solar) connects to the UK's public distribution network. It applies to any installation above 16 amps per phase (around 3.68kW on a single-phase supply, or 11kW on three-phase).

It replaced the older G59 standard in April 2019. The current version, G99 Issue 2, was updated in March 2025 and brought in mandatory requirements for electricity storage systems.

For commercial solar, the practical implication is straightforward: every commercial installation needs a G99 application, and it must be approved before the system can be energised. This is not optional, and it is not the installer's problem alone. The timeline affects your project directly.

G99 vs G98: the short version 

G98 ("connect and notify") applies to systems below 16A per phase (domestic scale). Your installer connects the system, then notifies the DNO within 28 days. No pre-approval needed. G99 works differently: you need written DNO approval before the system connects to the grid. The threshold is based on your inverter's AC export capacity, not the peak panel capacity. A 100kWp array with inverters configured below the threshold still requires a G99 application. 

Who needs a G99 application?

In practice: any UK business installing a commercial solar system. The 16A threshold is low enough that it captures almost all commercial PV. A relatively small 50kW three-phase system sits well above the limit. So does anything with battery storage where the combined export capacity exceeds the limit.

You need a G99 application if any of the following apply to your project:

• Total inverter AC output exceeds 3.68kW on single phase, or 11kW on three phase

• The system includes battery storage with grid export capability

• You are adding generation to an existing installation that takes the total above the threshold

The threshold is based on what the inverter is configured to export, not what the panels can produce. A system designed primarily for self-consumption with minimal export still needs G99 if the inverter capacity exceeds 16A per phase.

What does a G99 application actually require?

A typical application is submitted in stages. The core documents required for the initial submission are:

1. The Standard Application Form (G99 SAF)

Parts 1 to 3 must be completed at submission. Parts 4 and 5 can follow before commissioning, though for systems rated at 150kW or above, Part 4 is required at the initial submission stage. The form captures:

• Site address and connection point details

• Generating unit type (inverter-connected solar counts as a Power Park Module)

• Installed capacity and export capacity

• Single-line diagram of the electrical installation

• Proposed commissioning date

2. Single-line diagram

This is the most common reason applications get held up. The diagram must show the full electrical arrangement: panels, inverters, protection devices, metering, connection point, and export limitation equipment if applicable. Incomplete or incorrectly formatted diagrams can easily add two to four weeks to the process while the DNO requests corrections.

3. Inverter datasheet and type-test certificate

Every inverter must hold a valid ENA type-test certificate. Most major manufacturers such as SolarEdge, Solis, SMA and Sungrow, certify all their UK-market products.

4. Protection Settings Schedule

This sets out the protection relay values: voltage thresholds, frequency limits and trip times that the inverter will operate within. A blank schedule, or one with values outside the permitted ranges, is an automatic rejection trigger

5. Power Generating Module Document (PGMD)

For systems where the module is not fully type-tested, a PGMD is required. This is a compliance summary that tracks how the generation equipment meets G99 requirements through all project stages. The DNO keeps this on file and expects it to be updated as the project progresses.

6. Export limitation details (where applicable)

If the system uses Active Network Management (ANM) or a G100 export limitation scheme this must be declared and documented in the application.

The five most common reasons G99 applications are rejected or delayed 

1. Incomplete single-line diagram: missing protection devices or metering arrangement.

2. Inverter not on the ENA type-test register.

3. Protection Settings Schedule left blank or with out-of-range values.

4. Failure to declare existing generation at the connection point.

5. No export limitation details where the system uses ANM or G100. 

   
   

The G99 process: what happens and in what order

The application follows a structured sequence. Understanding it matters because several stages run in parallel with design and procurement decisions.

Stage 1: Pre-application enquiry

Before submitting a formal application, an experienced installer will check the local network capacity using DNO heat maps and publicly available connection data. This identifies whether the site is on a constrained part of the network, which affects both approval likelihood and timeline. Most DNOs offer a pre-application enquiry service, often free, that gives an indicative capacity figure and flags any likely reinforcement requirements before any fees are paid.

Stage 2: Formal application

The full application is submitted to the relevant DNO via their online portal. Each of the nine main DNOs in Great Britain, plus several Independent Distribution Network Operators (IDNOs) on private networks and industrial estates, operates its own submission system. The DNO is required to acknowledge receipt within 10 business days.

Application fees vary by DNO and connection voltage, but as a guide:

• Sub-200kW connections: approximately £350 to £500

• 200kW to 1MW: approximately £750 to £3,000

• Above 1MW: £3,000 to £15,000 or more

These fees are non-refundable, regardless of outcome. 

Stage 3: Technical review

For straightforward low-voltage connections, the DNO will typically respond within 45 calendar days. In practice this often runs to 8 to 12 weeks, and longer if the network is constrained or the site requires a formal capacity study. Systems above 250kW almost always trigger a full capacity study, which can extend the review to 16 to 26 weeks.

If the DNO requests additional information during the review, such as a corrected diagram or updated protection settings, the 45-day clock resets. This is the single biggest cause of timeline slippage. A complete, accurate submission first time avoids it.

Stage 4: Connection offer

If the network can accommodate the system, the DNO issues a connection offer specifying the export limit, any technical conditions, and the connection charge. The applicant typically has 30 to 90 days to accept and pay. Where the network requires reinforcement, the DNO will quote the reinforcement cost as a condition of connection. That cost can be substantial on constrained networks.

Stage 5: Commissioning and witness testing

Once installed, the system is commissioned against G99 technical requirements. For systems above 50kWp, the DNO may require a witnessed commissioning test, where a DNO representative or approved engineer attends to verify protection settings and confirm the system behaves correctly during grid fault simulations. Witness testing is normally chargeable and the fee varies by DNO. Due to the large number of installation’s Eden installs per year, for multiple DNO’s we are approved to undertake our own Witness tests on the DNO’s behalf, which saves both time and cost on each project.

After commissioning, the installer submits the G99 commissioning form (Stage 2 notification) to the DNO, confirming the system has been connected as approved and the system can then be energised.

How long does a G99 application take?

The honest answer depends on system size and network conditions. These are the realistic ranges:

• Up to 250kWp on an unconstrained network: 6 to 8 weeks end-to-end from application submission to approval

• 250kWp to 1MW requiring a capacity study: 6 to 18 weeks

• Sites needing network reinforcement: 3 to 12 months, sometimes even longer

These timelines run from the point of a complete, accepted application. An incomplete submission restarts the clock. A pre-application enquiry does not start the clock at all.

The practical implication for project planning: G99 should be submitted as soon as the system design is finalised, ideally in parallel with planning permission and structural surveys, not after them. Businesses that treat G99 as the final step in the process consistently experience the longest delays.

Who is responsible for the G99 application?

The technical and operational responsibility sits with the installer, though ultimate legal responsibility for connection compliance actually remains with the business as the site owner. A competent commercial solar contractor handles the entire G99 process: design documentation, form preparation, DNO liaison, protection settings, commissioning test coordination, and Stage 2 notification. The business does not typically contact the DNO directly, but because non-compliance directly affects the site’s connection point, ensuring proper approval is critical to protecting the asset owner.

What the business does need to provide: a signed letter of authority authorising the installer to act on their behalf, site access for any DNO site visit, and the MPAN (Meter Point Administration Number) for the connection point. For a PPA / funded installation, the legal agreement may also need to be in place before the DNO will issue a connection offer.

The quality of the installer's application matters more than most businesses realise. An experienced team will have submitted dozens of G99 applications across different DNO regions. They know which DNOs run tight and which run slow, which networks have capacity constraints, and what each DNO's reviewers’ flag most often. That local knowledge compresses the timeline while ensuring the business avoids any regulatory or operational pushback on their grid connection.

How Eden Sustainable handles the G99 process

Over ten years and more than 300 commercial and industrial solar installations, we have submitted multiple G99 applications to every DNO. The G99 process is part of every project from the design stage, not something we pick up after planning permission has been granted.

Before any application goes in, we run a pre-application check against the DNO's heat map data to identify any network constraints that might affect the connection offer. If a site sits on a constrained network, we know before the client commits. We can then design around it, whether that means adjusting system size, specifying export limitation, or advising on a realistic timeline.

We also manage the commissioning and witness testing stage, including DNO liaison on witness test scheduling. On constrained networks, scheduling alone can take several weeks to arrange. Nothing about the G99 process should come as a surprise on a well-run project. If it does, that is a planning failure, not a regulatory one.

Frequently asked questions 

Can I install commercial solar without a G99 application?

No, installing or energising commercial scale solar without G99 approval is a breach of distribution licence conditions. The only exception is preparatory works that do not involve connecting generation to the grid. The connection cannot legally go live without the DNO's written acceptance.

What happens if my G99 application is rejected?

Rejection usually means one of four things: the documentation was incomplete, the network at that connection point lacks capacity for the proposed export or there is an undeclared existing installation at the connection point. The first and last are avoidable. Network capacity rejection typically gives you options: accept a reduced export limit, agree to Active Network Management, fund reinforcement works yourself, or resize the system. Each route has different cost and timeline implications.

Does a zero-export system still need G99?

Even with export set to zero, the DNO needs to assess the installation because the generating equipment is connected in parallel with the network. The G100 export limitation process may simplify the application, but it does not remove the requirement.

Does the G99 application cost anything?

The application fee is paid to the DNO and is non-refundable. For a typical commercial rooftop system below 200kW, expect to pay £350 to £500. Larger systems or those requiring a network capacity study will naturally cost more. Where the network requires reinforcement, the reinforcement cost is quoted separately in the connection offer. That is where costs can become significant for constrained sites.

How does G99 interact with planning permission?

They run on separate tracks and neither is a prerequisite for the other. In practice, the most efficient approach is to progress both in parallel. Planning permission and G99 together determine when installation can begin. If G99 approval is delayed, even a project with full planning consent cannot connect. Treating them as sequential rather than parallel is one of the most common timeline mistakes in commercial solar projects.

Talk to us about your project

If you are planning a commercial solar installation and want to understand the G99 timeline for your specific site, we are happy to give you a straightforward assessment, including a pre-application check of your local network capacity. No forms, no commitment.

Contact us