G99 Application Guide: UK Solar Grid Connection — What Homeowners & Businesses Need to Know (2026)

What is a G99 application, and why does it play such a critical role in solar installations across the UK? In short, a G99 application is the pre-installation grid approval that UK homeowners and businesses need for systems above 3.68 kW per phase. It directly affects your installation timeline, system design and overall cost in 2026. This guide explains the process, timelines, costs and how to avoid the delays that catch most projects out.

A G99 application appears during the planning stage of solar installation, and homeowners often remain unsure about its importance. It is not just an administrative step. It is a technical approval process that determines whether your system can safely interact with the electricity grid. The difference between a smooth installation and a delayed one often comes down to how well this stage is handled. Experienced installers who prepare clean, accurate applications are far less likely to face delays, rejection or system redesign requests from the DNO. See also our DNO solar application guide for how the broader grid connection process works.

What Is a G99 Application?

A G99 application is a formal approval process required by UK electricity networks before larger energy-generating systems can connect to the grid. This includes solar panels and battery storage. It is part of the UK’s Engineering Recommendation G99 framework, designed to protect grid stability as more renewable energy systems are installed across the country. Unlike smaller systems operating under G98, which can be installed and notified afterwards, G99 systems require explicit permission before any installation work begins. This distinction is critical, because it means your entire installation timeline depends on G99 approval being in place first. No approval, no installation.

G99 vs G98: understanding the threshold

The UK grid connection framework divides generating systems into two categories. G98 covers systems up to 3.68 kW per phase — these require only post-installation notification to the DNO and carry no application fee. G99 applies to everything above this threshold and requires pre-installation approval. The 3.68 kW per phase limit matters most on a single-phase supply, which describes the majority of UK homes. There, any system above 3.68 kW total — including the discharge capacity of battery storage where applicable — falls under G99 rather than G98.

What the DNO assessment covers

When a G99 application is submitted, the Distribution Network Operator assesses how your system’s export capacity could affect voltage levels, network stability and neighbouring properties. The central concern is voltage rise: if multiple solar systems in the same area are exporting simultaneously, local voltage can exceed safe limits. The DNO’s assessment determines whether your system’s contribution creates a problem. If so, it sets out what mitigation is required, typically an export limitation rather than outright refusal.

When Do You Need a G99 Application?

The requirement for a G99 application depends on your system’s output, phase configuration and in some cases battery capacity. Many homeowners assume G99 applies only to large or commercial systems. In practice, with increasing system sizes and growing battery adoption, G99 is now the norm for a large proportion of modern UK residential installations rather than the exception. The clearest indicator is the 3.68 kW per phase threshold, but several common scenarios push systems into G99 territory even when the panel count appears modest.

Systems that typically require G99

• Solar systems of 4 kW or larger on a single-phase supply
• Any system where the inverter output rating exceeds 3.68 kW per phase
• Battery storage systems with significant discharge capacity, including all Tesla Powerwall 3 installations (11.04 kW inverter)
• Upgrades or expansions to existing solar systems that push total output above the threshold
• Any installation on a three-phase electrical supply where combined output exceeds 11.04 kW

Common scenarios that trigger G99 unexpectedly

The most common surprise is battery storage. Many homeowners add a battery to an existing G98 system and discover that the combined inverter output now requires G99 approval. This is particularly common with high-capacity batteries such as the Tesla Powerwall 3, which carries an 11.04 kW inverter regardless of the connected solar array size. Similarly, homeowners upgrading an existing system to add more panels can inadvertently cross the threshold. A competent installer confirms G98/G99 status before finalising system design, not after.

How the G99 Application Process Works

The G99 process involves several stages, each of which plays a role in ensuring your system is safe and compatible with the local grid. While the DNO assessment is the most time-consuming stage, the quality of the application submitted to them determines how quickly and smoothly that assessment proceeds. A well-prepared application includes complete technical documentation, accurate system specifications and evidence that grid constraints have been considered in the design. That significantly reduces the likelihood of requests for additional information or system redesign.

The six stages of a G99 application

1. System design: the installer designs the system based on energy needs and roof constraints, while accounting for known grid capacity in your area
2. Technical documentation: the installer prepares the complete application pack including inverter specifications, protection settings, electrical diagrams and site layout
3. Submission to DNO: the application is submitted to your local Distribution Network Operator for assessment
4. DNO network assessment: the DNO evaluates how your system will interact with the local grid and determines whether approval, export limitation or further analysis is required
5. Approval or conditions: the DNO issues a connection offer. This may be unconditional, export-limited, or subject to additional technical requirements
6. Installation and commissioning: once approval is in place, installation proceeds. The system must be commissioned and tested in accordance with the approved specification

What happens if the DNO imposes export limits

Export limitation is a common outcome rather than a problem. The DNO may cap how much electricity your system can export to the grid at any one time. For example, it might limit export to 3.68 kW even if the system can generate more. This doesn’t prevent the system from operating; it simply means generation above the export cap must be used on-site or stored in a battery. For homes with high daytime consumption or battery storage, an export limit often has minimal practical impact. For homes that export heavily, it may affect the financial return from Smart Export Guarantee payments.

How Long Does G99 Approval Take?

G99 approval timelines are one of the main planning constraints in any solar project that requires it. Because approval must be in place before installation begins, the DNO timeline effectively sets the earliest possible installation date. For this reason, experienced installers initiate the G99 process as early as possible, ideally immediately after system design is finalised, before scaffolding or materials are ordered. Building the G99 timeline into project planning from the start prevents the most common cause of programme delays. That is discovering that a 10-week approval process hasn’t started until two weeks before the planned installation date.

Typical timelines by system type

What causes delays

Delays are almost always caused by avoidable issues rather than the process itself. Missing technical documentation, incorrect protection settings, and system specifications that don’t match the submitted diagrams are all common triggers. So are designs that push local grid capacity beyond what the DNO can accommodate without remediation. Together, these are the most common sources of additional correspondence and resubmission. Experienced installers who understand DNO requirements from the outset are significantly less likely to encounter them.

What Does a G99 Application Cost?

The cost of a G99 application varies depending on your DNO, the complexity of your system and whether any additional technical studies or infrastructure work are required. In many straightforward residential cases, the process costs nothing beyond the installer’s time to prepare and manage the application. In more complex cases — particularly for larger systems, commercial installations or properties in areas with constrained grid infrastructure — additional costs can arise. A thorough pre-application assessment helps identify likely costs before they appear as surprises in the project budget.

Application and engineering costs

Where the real financial risk lies

The application fee itself is rarely the significant cost. The real financial risk comes from delays or system redesigns caused by poor application preparation. A project delayed by eight weeks due to a resubmission request means eight additional weeks of electricity bills before the system is generating. A system redesign required by the DNO after submission can mean reordering equipment and rescheduling installation. Both of these outcomes are largely preventable by experienced handling of the application.

Common Mistakes That Delay G99 Approval

The G99 process itself is not inherently complex, but it requires precision. DNOs issue straightforward approvals when applications are complete, accurate and demonstrate that the system design has taken grid constraints into account. Delays almost always stem from avoidable errors in documentation, design or timing rather than from DNO intransigence. Understanding the most common mistakes helps both homeowners and installers avoid them.

Documentation and design errors

• Incorrect or inconsistent system specifications — inverter ratings, panel count or array capacity that don’t match across different sections of the application
• Missing protection settings or incompatible relay configurations that don’t meet the DNO’s technical requirements
• Designing systems that exceed local grid capacity without considering export limitation as part of the design from the outset
• Using inverters or equipment not on the DNO’s pre-approved or certified equipment lists

Timing and process errors

• Submitting the application too late in the project timeline, creating pressure if approval takes longer than expected
• Failing to respond promptly to DNO requests for additional information, which can reset the assessment clock
• Proceeding with installation before written approval is in hand, which can create compliance issues and grid connection refusal

Why Installer Experience Matters for G99

The quality of your installer is the single biggest factor in how smoothly the G99 process runs. Experienced installers have a working understanding of each regional DNO’s specific requirements and preferences. That lets them design systems and prepare applications in ways that minimise requests for additional information or design changes. They also manage the timeline proactively, initiating the process early and maintaining direct communication with the DNO throughout. For homeowners, the result is a more predictable programme and fewer surprises.

What an experienced installer handles

• Designing systems within local grid constraints, accounting for known capacity issues in your area
• Preparing complete, accurate technical documentation that meets the specific requirements of your DNO
• Submitting a compliant application first time, avoiding the back-and-forth of incomplete submissions
• Communicating directly with the DNO to resolve queries quickly
• Managing the approval timeline so it integrates with scaffolding, materials and installation scheduling

Questions to ask about G99 before you proceed

Ask your installer: who handles the G99 application, and at what point in the process is it submitted? Is the application fee included in my quote? Do you have experience with my local DNO’s specific requirements? What happens if the DNO requests a design change? A reliable installer answers all of these clearly. Hesitation or vague responses about the G99 process are a meaningful signal about overall project management capability.

Conclusion

The G99 application is a critical part of the solar installation process in the UK. When managed correctly, it runs in the background while the project progresses. When managed poorly, it becomes the bottleneck that delays everything else. For homeowners and businesses planning a solar installation in 2026, the key takeaway is simple. Start the G99 process as early as possible and submit it with complete, accurate documentation. It should also be managed by an installer with direct experience of your local DNO’s requirements.

Solar4Good handles the entire G99 process as a standard part of every relevant installation, including documentation, submission and DNO correspondence. The timeline and any associated costs are confirmed in the written quote before work begins, so there are no surprises once the project is underway.

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