Grid connection is one of the four engineering flags screened in every Stage1Energy feasibility dossier. For UK commercial rooftop PV, the route almost always comes down to two Energy Networks Association documents: G98 for smaller installations and G99 for larger ones. Getting this wrong at the outset is one of the most common reasons a promising roof turns into a delayed, over-budget project.
What G98 and G99 actually govern
G98 and G99 are not planning permissions and they are not building regulations. They are the connection standards that distribution network operators (DNOs) use to decide how a generator connects to the low-voltage network.
G98 applies where the total installed capacity is within the deemed limits for a given supply arrangement. For a typical three-phase commercial supply, that means up to 3.68 kW per phase — or 16 A per phase where capacity is expressed in current. Connection under G98 is handled through a notification process: you inform the DNO once the installation is commissioned, provided the works meet the standard.
G99 applies above those thresholds. A commercial rooftop array of any meaningful size — think warehouse, factory, or retail park — will almost certainly require a G99 application. That means submitting technical data to the DNO, waiting for assessment, and potentially negotiating connection offers, reinforcement costs, or export limitations before you can proceed to installation.
The distinction matters because G99 introduces time, cost, and uncertainty that G98 does not. A 200 kWp warehouse roof is not a paperwork exercise; it is a grid engineering question.
How DNOs assess a G99 application
When you submit under G99, the DNO reviews whether the local network can absorb your export and whether the connection point can handle the fault level and voltage rise associated with your inverter capacity. For commercial sites, common outcomes include:
- Unrestricted connection — the network has headroom and you proceed on standard terms.
- Connection with conditions — export may be limited, or you may need additional protection equipment.
- Reinforcement required — the DNO identifies network upgrades, sometimes at significant cost and with long lead times.
- Refusal or deferral — rare on rooftop PV, but possible where the local feeder is already constrained.
None of this is visible from a satellite image alone. But a good commercial solar feasibility assessment can screen the likely route from declared supply data, proposed capacity, and DNO area — flagging whether G99 is required and naming the next step before you spend on structural surveys or detailed design.
Why commercial rooftops almost always need G99
A single commercial PV module is typically 400–600 Wp. A modest 100 kWp array might use 180–250 panels. On a three-phase supply, that capacity dwarfs the G98 per-phase threshold many times over.
Even where a site has multiple meters or a complex supply arrangement, the aggregate export capacity is what the DNO assesses. Splitting an array across phases does not circumvent G99 if the total installed capacity exceeds the deemed limit. Installers sometimes discover this late — after layout, pricing, and board approval — when a G99 application reveals export constraints or reinforcement costs that were not in the original business case.
Screening the grid route at feasibility stage protects against that surprise. Our methodology describes DNO screening in every dossier; G98/G99 thresholds and programme allowances are named in the report, with a next step rather than a vague “check with the DNO” note.
Timelines and programme risk
G99 lead times are inconsistent across DNOs and have lengthened in some areas as connection queues grow. For programme planning, three to six months from application to connection agreement is a reasonable working assumption for a typical commercial rooftop — longer where reinforcement is involved.
That timeline sits alongside planning (often straightforward under permitted development for rooftop PV), procurement, and installation. If your business case assumes energisation within twelve weeks of board approval, a G99 requirement can invalidate the programme before a single panel is ordered.
A feasibility dossier does not replace the DNO application. It tells you whether G99 is in scope, what it likely means for your programme, and whether the grid route is a low, medium, or elevated risk — so you can decide whether to pursue the roof at all. See the example report for how the DNO flag appears alongside wind, structural, and planning screening.
What to provide for an early grid screen
The more accurate your supply data, the more useful the screen. Useful inputs include:
- Declared supply capacity — from the MPAN, half-hourly data, or a recent bill.
- Number of phases and whether the supply is HV or LV.
- Proposed array capacity — from a layout based on usable roof area.
- DNO identity — which of the GB distribution operators serves the site.
Where supply data is uncertain, the dossier states its assumptions explicitly. That is preferable to silent defaults that collapse when the DNO responds.
For a quick first read without committing to a full dossier, the free screening gives a verdict and layout on your building in three working days. The full site assessment adds the four engineering flags — including DNO — plus financials and sourced calculation workings.
G98/G99 screening in the feasibility dossier
In a Stage1Energy dossier, the DNO flag answers three questions:
- Does this array require G99? — almost always yes for commercial scale.
- What is the screening risk level? — based on capacity relative to supply and known DNO constraints in the area.
- What is the named next step? — typically “submit G99 application” with a suggested programme allowance.
This is screening, not approval. The DNO makes the final determination. But screening early — alongside wind, structural, and planning flags — gives estates teams and advisers a complete engineering picture before capital is committed.
If you are evaluating a commercial roof for solar, start with the grid route. G98 and G99 are not arcane compliance labels; they are the gate between a financial model and a connected asset.