Solar Carport G99: How the DNO Process Works

What G99 is

Engineering Recommendation G99 is the UK standard governing grid connection of generation up to 50 MW. It replaces the older G59 standard (which now only applies to legacy connections). All new UK commercial solar carports submit a G99 application via the local Distribution Network Operator (DNO).

UK DNO regions and contacts

Six UK DNO regions: UK Power Networks (UKPN) covers Greater London + South East + East of England; National Grid Electricity Distribution (NGED) covers West Midlands + South West + East Midlands + Wales (formerly Western Power Distribution); Electricity North West (ENW) covers the North West; Northern Powergrid covers the North East + Yorkshire; Scottish & Southern Electricity Networks (SSEN) covers South East + Scotland (Northern); SP Energy Networks (SPEN) covers Scotland (Central/Southern); NIE Networks covers Northern Ireland.

The G99 process step-by-step

Process: (1) Pre-application — DNO capacity check (free, ~5-10 working days); (2) Full G99 application submitted — DNO reviews technical pack (typically 4-12 weeks); (3) DNO connection offer issued — typically 90 days for sub-1MW, 150-200 days for larger; (4) Offer acceptance — typically 90 days to accept; (5) Witness test on switch-on — DNO field engineer present at first energisation.

Connection costs

Two cost components: (1) Standard connection fee — typically £2,000-£12,000 for sub-1MW connections, scaling with capacity; (2) Reinforcement contribution — variable, zero for most sites under 500 kVA but can run £20-£250k+ for sites where upstream substations need upgrade. Pre-application capacity check identifies reinforcement risk early.

Common G99 gotchas

Five frequent issues: (1) Site capacity shortfall — your import capacity isn't large enough; usually solvable by export-limiting (G100) rather than capacity upgrade; (2) Phase imbalance on three-phase supplies — inverters must be balanced across phases; (3) Protection relay configuration — site protection (G99 relay) must be type-tested; (4) Lock-out documentation — DNO requires isolation procedure for maintenance; (5) Witness test scheduling — DNO field engineers are constrained; book early.

G100 export-limited connections

If your DNO has limited spare export capacity, the connection offer may be conditional on export limitation under Engineering Recommendation G100. Common for sites above 500 kVA in constrained network areas (West Midlands, Greater London). Solution: implement a hard export limiter on the inverter or via a dedicated controller. Doesn't affect site self-consumption — only how much surplus can flow to grid.

DNO typical timelines in 2026

Indicative ranges only: timescales vary by operator, network area and scheme size, and must be confirmed with the network operator. A standard application commonly takes in the region of 60 to 80 working days, and a complex one 120 to 160 working days. Build planning realistically — DNO timing is the single biggest variable in carport project delivery.

Who handles the G99 application

Whoever runs the application, the technical steps are: (1) the pre-application capacity check; (2) the full G99 application submission; (3) DNO clarification responses; (4) protection relay specification; (5) witness test scheduling and attendance; (6) post-energisation paperwork. The site owner signs the DNO connection agreement; ask the installer to confirm in writing which of the technical steps it is taking on.

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