The process, timescales and costs UK businesses should expect before a commercial solar system can legally export power to the grid.
G99 is the Engineering Recommendation that governs how generating equipment above 16A per phase — roughly 3.68kW single-phase or 11.04kW three-phase, which covers virtually every commercial solar system — connects to the UK electricity distribution network. Unlike the simpler G98 “connect and notify” process for small domestic systems, G99 requires formal approval from your local Distribution Network Operator (DNO) before the system can be energised. For most commercial installs, expect an application cost of roughly £500 to £8,000 and a timeline of 8 to 16 weeks where local network capacity is available, extending to 6–12+ months where the network needs reinforcement. Your installer normally manages the whole process, but understanding the stages helps you plan a realistic project timeline.
Every commercial solar quote eventually reaches the same practical question: how does the system actually get permission to send power back to the grid? That’s what G99 covers. It’s not a formality that can be skipped or rushed — without it, a system can generate electricity but can’t legally export it, and in some cases can’t be commissioned at all.
This guide sets out what G99 is, when it applies, the steps involved, realistic timescales by project size, and what the DNO fees typically look like — so you can build an accurate project timeline before committing to an installation date.
Treating G99 as something to sort out once the system is designed and ready to install. The application can, and should, be submitted as early as possible — ideally as soon as the system size and inverter specification are agreed — because the DNO’s response time is usually the longest single item on the whole project timeline, not the panel installation itself.
G99 — formally Engineering Recommendation G99, published by the Energy Networks Association — sets the technical standard for connecting generating plant, including commercial solar PV, to the UK electricity distribution network. It exists to protect the grid: without it, a DNO has no way of knowing whether a new generator will destabilise local voltage, overload a feeder, or fail to disconnect safely during a fault.
Whether your business needs G99 or the simpler G98 process comes down to inverter capacity rather than panel wattage. Systems with an inverter rated at 3.68kW or under per phase (single-phase), or 11.04kW total on three-phase, generally qualify for G98 “connect and notify” — install first, notify the DNO within 28 days. Anything above that threshold needs G99 approval before the system can be energised, which in practice means almost every commercial installation.
Covers inverters up to 3.68kW single-phase or 11.04kW three-phase. Your installer notifies the DNO after commissioning rather than seeking approval beforehand, and approval typically confirms within around 10–15 working days. Very few commercial systems qualify, since most business inverters exceed this threshold by design.
Covers any system above the G98 threshold — which includes practically every rooftop commercial array and all ground-mounted or large-scale installations. The DNO must review and approve the connection before your system can be energised. Applications are submitted using the ENA’s standard application form, alongside supporting technical documentation covering the inverter specification, protection settings and single-line diagram.
- Completed ENA Standard Application Form
- Single-line diagram (SLD) of the proposed installation
- Inverter datasheets confirming current G99 type-test certification
- Proposed protection relay settings
- Site plan and existing supply details (MPAN, capacity, phase configuration)
The process is administrative and technical rather than physically disruptive — the panel installation itself can often proceed in parallel once a connection offer is accepted, but the system cannot be switched on and exporting until G99 commissioning is complete.
In Essex, the DNO is UK Power Networks. Its capacity heat maps are a useful early planning tool — areas shown as constrained are worth flagging before committing to a system size, since they signal a longer or more expensive connection process ahead.
Timescales vary considerably by system size and local network capacity, which makes G99 one of the harder parts of a project to schedule precisely. The DNO has a statutory period of up to 45–60 working days to issue a response, but real-world timelines often run longer once network studies and any reinforcement works are factored in.
| System scenario | Typical timeline | Notes |
|---|---|---|
| Fully type-tested, up to ~50kW three-phase | Weeks | Streamlined fast-track process |
| Up to ~250kWp, capacity available | 8–16 weeks | Standard application and study route |
| Larger systems needing a capacity study | 16–26 weeks | Full technical assessment required |
| Sites needing network reinforcement | 6–12+ months | Depends on scale of local network works |
| Above 1MW, constrained network | Up to 18–24 months | May involve transmission-level (NESO) assessment |
Some DNO areas are notably more constrained than others for new solar export, particularly parts of the South West, and some rural feeders have very limited spare capacity. It’s worth getting an indicative timeline for your specific postcode from your installer before finalising a project schedule, rather than relying on national averages alone.
G99 costs fall into a few distinct categories, and the total depends heavily on whether the local network has spare capacity or needs reinforcement to accommodate your system.
| Cost item | Typical range | When it applies |
|---|---|---|
| Application/study fee | £350–£2,000 | Most sub-200kW commercial applications |
| Capacity study fee | £2,000–£8,000 | Systems above roughly 250kW |
| Witness testing fee | £500–£1,500 | Systems above roughly 50kWp |
| Network reinforcement | £15,000–£500,000+ | Only where the local network needs upgrading |
For most SME commercial projects in the 100–500kWp range, a realistic budget for DNO-related costs sits around £3,000–£8,000, assuming no significant reinforcement is needed. Network reinforcement is the genuine wild card — it’s worth requesting an indicative figure during the pre-application phase, since in some cases downsizing the system or accepting an export limit is more economical than funding the network upgrade.
Not every connection offer grants full, unrestricted export. Where the local network is close to capacity, the DNO may cap your export at a level below your system’s nameplate rating rather than reject the application outright — this is one of the most common outcomes for mid-sized commercial systems on constrained feeders.
Your system still generates its full potential; any excess beyond what you use on-site and the agreed export limit is simply curtailed by the inverter rather than exported. For businesses with high daytime self-consumption — most commercial premises operating standard hours — an export limit often has minimal financial impact, since the bulk of the value comes from displacing imported electricity rather than exporting surplus. For businesses with lower daytime usage, such as warehouses running light shifts, curtailment can mean a meaningful amount of generation goes unused.
Where export limits bite, right-sizing the system to your typical load and adding battery storage to time-shift excess generation into higher-use periods are both practical ways to reduce lost generation without paying for network reinforcement.
Most G99 delays trace back to a small number of avoidable issues, and knowing them in advance can save weeks on a project timeline.
- Specifying an inverter not on the current G99 type-test register — confirm certification before finalising the design
- Submitting the application late, after installation is already scheduled — apply as soon as the system design is agreed
- Incomplete documentation, particularly missing single-line diagrams or protection settings
- Underestimating timelines on constrained networks — check DNO capacity heat maps early
- Assuming any brand of inverter will pass — only currently certified equipment from established manufacturers should be specified
- Inverter holds a current, valid G99 type-test certificate
- Protection settings match the connection offer once received
- Single-line diagram accurately reflects the as-designed system
- Application submitted as soon as the system design is agreed, not after
- Realistic project schedule built around DNO response times, not installation time alone
- Local network capacity checked via DNO heat maps or pre-application enquiry
- Application and study fees budgeted for based on system size
- Indicative reinforcement costs requested if the network looks constrained
- Export limit scenario understood, and self-consumption profile reviewed against it
- G99 applies to virtually all commercial solar — anything above roughly 3.68kW single-phase or 11.04kW three-phase.
- Approval is needed before a system can be energised, unlike the simpler G98 “connect and notify” route for very small systems.
- Typical timelines run 8–16 weeks where capacity is available, extending to months where network reinforcement is required.
- Budget roughly £3,000–£8,000 in DNO costs for a typical SME commercial project, with reinforcement costs as the main wild card.
- Submit the application early — it’s usually the longest single item on the project timeline, so starting late costs real weeks.
Let us manage your G99 DNO application
We handle the full grid connection process — application, protection settings and witness testing — as standard on every commercial install, with no separate admin fee.
