G99 Grid Connection for Commercial Solar: A Practical Guide

G98 and G99 are engineering standards published by the Energy Networks Association that set out how generators, including solar panels and batteries, connect to the electricity distribution network. Which one applies depends on how much generation is installed at the premises.

• G98: covers small, type-tested generation up to 16 amps per phase. That is 3.68kW on a single-phase supply and 11.04kW on a three-phase supply. The installer fits and commissions the system, then notifies the DNO, normally within 28 days.
• G99: covers everything above the G98 limits, with no upper limit. The DNO must assess and approve the connection before the system is switched on.

The threshold applies to all generation and storage at the premises combined, not to each inverter. Adding a battery to an existing solar system can therefore move a site into G99.

Most commercial properties have three-phase supplies, and almost all commercial solar systems exceed 11.04kW. In practice, that means G99 is the normal route for businesses, and the application belongs in the project plan from the start rather than at the end.

Great Britain is divided into areas served by six distribution network operator groups, and your application goes to whichever one owns the network your building connects to. Your installer will normally identify the DNO from your address or meter number, but it helps to know who they are:

• National Grid Electricity Distribution: the Midlands, South West England and South Wales
• UK Power Networks: London, the South East and East of England
• Northern Powergrid: the North East, Yorkshire and northern Lincolnshire
• Electricity North West: North West England
• SP Energy Networks: central and southern Scotland, Merseyside, Cheshire and North Wales
• Scottish and Southern Electricity Networks: northern Scotland and central southern England

Each DNO publishes its own application forms, guidance and fees, and many offer maps showing where the network has spare capacity. Network capacity varies widely from street to street, so two similar buildings in the same town can receive very different connection offers.

A G99 application follows a broadly similar sequence across all DNOs:

1. Design: the installer designs the system, including inverter sizes, protection settings and any export limit.
2. Application: the installer submits the relevant G99 forms with technical details of the equipment and site. Smaller installations may use a simplified form; larger ones need a full application.
3. Assessment: the DNO studies the effect on its local network, including voltage and fault levels.
4. Connection offer: the DNO issues an offer setting out any conditions, export limits, required works and costs. For a full application, it has up to 45 working days to do so.
5. Acceptance: you accept the offer within its validity period and pay any charges.
6. Installation and commissioning: the system is installed and commissioned. Larger systems may need the DNO to witness testing.
7. Final notification: the installer submits commissioning documents, and the DNO records the connection.

The system must not be switched on to export until the DNO has approved it. Connecting without approval can breach your connection agreement and the electricity safety regulations.

For a full G99 application, the DNO has up to 45 working days, roughly nine weeks, to issue a connection offer once it has a complete application. That clock only starts when the application is accepted as complete, so missing information resets the wait.

Real timescales vary:

• Straightforward applications: small commercial systems with export limits on unconstrained networks can be approved well within the maximum.
• Typical commercial projects: six to ten weeks from submission to offer is common.
• Constrained networks or large systems: where reinforcement is needed, design studies and works can extend the timeline to many months.

Add time for any required DNO works after you accept the offer, and for witness testing on larger systems.

The practical lesson is that G99 can take longer than the installation itself. A 100kWp rooftop system may take a week or two to install, but the grid application can take two months. Projects that leave the application until the design is final often end up with panels on the roof waiting to be switched on.

G99 costs fall into two parts: the application and assessment fee, and the cost of any network works the DNO requires.

Application fees for a full assessment typically range from around £500 to £2,500, depending on the DNO, system size and complexity. Many installers include the application in their quote, but some list it as an extra, so check.

Network works are where costs can escalate. If the DNO finds that your system would push voltage or fault levels beyond safe limits, it may require:

• Upgrades to the local transformer or cables
• New protection equipment at your site
• Changes to your connection arrangement

Reinforcement costs vary enormously, from nothing on an unconstrained network to tens of thousands of pounds or more where the local network is near capacity. Under current charging rules, the connecting customer contributes to the cost of works needed for the connection, and the offer will set out your share.

Because this is the least predictable cost in a commercial solar project, it should be identified before you sign a final contract. Our commercial solar cost guide explains how it fits into the overall budget.

When a network cannot accept your full export, the most useful tool is export limitation. The inverters, controlled by a meter or current clamp at your incoming supply, reduce output whenever export to the grid would exceed an agreed limit. The limit can be set at any level the DNO agrees, including zero.

Export limitation schemes follow a separate Energy Networks Association standard, G100, which sets out how they must be designed so the DNO can rely on them.

For most businesses the cost is small. A system sized to your daytime demand uses most of its output on site, so the electricity curtailed by an export limit is mainly surplus that would have earned only a few pence per kWh. Against that, an export limit can:

• Avoid or reduce reinforcement charges
• Speed up approval
• Allow a larger system than the network would otherwise accept

Ask your installer to request two options in the application or pre-application discussion: the full export you would ideally like, and an export-limited alternative. Seeing both offers side by side is the clearest way to judge whether reinforcement is worth paying for.

Batteries and some EV chargers count towards your total generation and storage capacity for G99 purposes, so adding them can change your application.

• Batteries: a battery that can export to the grid adds to the capacity the DNO assesses. Adding one to an existing system can trigger a new or revised application, even if the solar array was originally connected without difficulty.
• Battery charging from the grid: a battery that charges at night increases your import demand, which may need to be checked against your supply capacity.
• EV chargers: standard chargers add demand rather than generation, but vehicle-to-grid chargers can export and are treated as generation.

If storage or electric vehicles are likely within a few years, include them in the original application where possible. A single application covering solar, storage and future EV charging is usually quicker and cheaper than several separate ones, and it lets the DNO assess the whole site at once.

Export limitation can be applied to the combined system, so a battery charged from solar does not necessarily increase the export the DNO needs to approve.

Most G99 delays come from avoidable problems rather than the DNO's assessment. The most common are:

• Incomplete applications: missing single-line diagrams, equipment data or site details mean the application is returned and the 45-day clock restarts.
• Late submission: applying only after the system has been designed, contracted and scheduled for installation.
• Design changes: changing inverter sizes or adding a battery after submission, which may require a revised application.
• Uncertain supply details: not knowing the maximum import capacity of your supply, or whether it is single-phase or three-phase.
• Unclear ownership: on leased premises, confusion over who is the applicant and whether the landlord has consented.
• Expired offers: letting a connection offer lapse while the project is approved internally, so the application must start again.

A competent commercial installer should handle the application, but the business can help by supplying accurate supply details early, agreeing the final design before submission and setting aside time to approve the connection offer promptly when it arrives.

The order in which you do things decides whether G99 helps or hinders the project. A sequence that works well for most businesses:

1. Gather 12 months of consumption data and your supply details, including import capacity and phase
2. Get quotes and a preliminary design from installers
3. Choose an installer, agreeing that the G99 application is part of their scope
4. Submit the application early, with a full-export option and an export-limited alternative where appropriate
5. Make the installation contract conditional on an acceptable connection offer, or include a price adjustment clause for DNO charges
6. Accept the offer within its validity period and pay any charges
7. Schedule installation to finish when the connection is approved

The key point is step five. Making the contract conditional on the grid offer protects you from committing to a project whose economics change once the DNO responds. Our guide to comparing commercial solar quotes explains how to check that G99 is properly included in each proposal.

Before you sign, ask each shortlisted installer these questions about the grid connection:

• Is the G99 application included in your price, and are the DNO's fees included or extra?
• When will you submit the application, and what information do you need from us?
• Have you checked the DNO's capacity information for our area?
• Will you apply for an export-limited option, and what would it cost us in lost export?
• How will the contract handle reinforcement charges if the DNO requires them?
• Does the system need witness testing, and who arranges it?
• If we add a battery or EV chargers later, will a new application be needed?

Installers who regularly deliver commercial projects will answer these easily. Vague answers, or a suggestion that G99 is a formality to be dealt with after installation, are a warning sign.

For more on how system size affects the grid application, see our commercial solar sizing guide. When you are ready, compare quotes from up to three MCS-certified commercial installers experienced in G99 applications.