Every commercial rooftop PV project that exceeds G98 thresholds must secure agreement from the local distribution network operator (DNO) before connection. That process — typically a G99 application under ENA standards — is the subject of the grid flag in commercial solar engineering flags screening. It is also one of the longest and least predictable stages in a commercial solar programme.
What the DNO actually decides
The DNO — UK Power Networks, Northern Powergrid, SP Energy Networks, SSEN, Electricity North West, or National Grid in some areas — owns and operates the cables and substations that deliver power to your site. When you propose to export solar generation, the DNO assesses:
- Whether the local network can absorb your export without voltage rise or thermal overload.
- Whether your connection point can handle the fault contribution from your inverters.
- Whether reinforcement — new cables, transformer upgrades, or protection equipment — is needed.
- What export limitation, if any, applies to your connection agreement.
This is independent of planning permission and independent of your installer’s design. The DNO’s determination can constrain your array size, export capacity, or energisation date regardless of what the roof can physically accommodate.
For background on the G98/G99 threshold itself, see G98 and G99 explained. This article focuses on the application process once you know G99 applies.
The G99 application process in outline
While each DNO has its own portal and paperwork, the commercial rooftop journey typically follows these stages:
- Pre-application enquiry (optional) — some DNOs offer an informal capacity check before formal submission. Useful where network constraints are suspected.
- Formal G99 application — submission of technical data: proposed capacity, inverter specifications, connection point, single-line diagram, and export profile.
- DNO assessment — the operator reviews network data, sometimes requesting further information or a site visit.
- Connection offer — the DNO issues terms, including any reinforcement costs, export limits, or protection requirements.
- Acceptance and commissioning — once terms are agreed and the installation is complete, commissioning and energisation follow DNO procedures.
Timelines from submission to connection offer commonly run three to six months for a typical commercial rooftop. Reinforcement can extend that substantially. Our methodology describes DNO screening; programme allowances appear in the dossier so the business case does not assume instant connection.
What to prepare before applying
A DNO application needs more detail than a feasibility dossier provides — but the dossier tells you whether the application is worth preparing. Useful inputs at application stage include:
- MPAN and supply details — from the electricity bill or half-hourly meter data.
- Proposed gross capacity — from a layout on usable roof area.
- Inverter specification — make and model, with G99/Type Test certification.
- Connection point — main intake location and existing supply capacity.
- Export limitation strategy — if the DNO caps export below installed capacity.
Where inverter selection is not yet fixed, the feasibility dossier uses representative commercial specifications. The example report shows how the DNO flag presents the grid route alongside wind, structural, and planning screening.
Costs and business case risk
DNO connection costs are among the least predictable line items in a commercial solar budget. Outcomes range from:
- Standard connection charge — often a few hundred to a few thousand pounds for a straightforward LV connection.
- Export limitation — no capital cost, but reduced revenue if export is capped below generation.
- Reinforcement contribution — the DNO identifies network upgrades; costs can reach £50,000 or more on constrained feeders.
- Delayed energisation — programme cost rather than direct charge, but material to IRR on time-sensitive investments.
Commercial solar feasibility cannot quote DNO charges — only the DNO can. But it can flag elevated grid risk where capacity is large relative to supply, or where the DNO area has known constraints. That flag adjusts the verdict and programme assumptions before you commit to surveys and application fees.
Running DNO alongside other workstreams
Experienced developers often run the DNO application in parallel with planning (or PD verification), structural survey, and procurement. Parallel running saves time — but only if the feasibility verdict supports pursuit.
The sequencing logic:
- Feasibility screen — all four flags, financial verdict, layout. Free screening or full site assessment.
- Board or investment decision — based on the dossier, not an installer’s verbal assurance.
- Parallel submissions — DNO G99, planning (if needed), structural engineer, installer tender.
- Detailed design — once connection offer and surveys are in hand.
Skipping step one is how portfolios accumulate sunk cost on roofs that fail on grid grounds after planning and structural spend are committed.
DNO screening in the feasibility dossier
The grid flag in a Stage1Energy dossier is not a DNO application. It is a screened assessment of the likely connection route:
- G99 required? — yes for virtually all commercial arrays.
- Risk level — based on capacity relative to declared supply and area factors.
- Named next step — “submit G99 application; allow 3–6 months” is typical.
- Programme impact — reflected in the financial model where relevant.
This sits alongside wind and structural flags so the engineering picture is complete. For advisers packaging board papers, the DNO flag is the programme risk line that finance directors ask about first.
What screening does not replace
- It does not submit or manage the G99 application.
- It does not negotiate connection offers or reinforcement costs.
- It does not guarantee export capacity or connection date.
- It does not replace the DNO’s formal assessment.
It does tell you whether the grid route is a low-friction item or a material risk — before you spend on the application itself.
For commercial rooftops, the DNO is not an afterthought. Screen it at feasibility stage, build the timeline into your business case, and apply early once the verdict supports pursuit.