G99 is the Engineering Recommendation issued by the Energy Networks Association governing grid connection of commercial-scale embedded generation in Great Britain. Every solar PV system above 16 amps per phase (about 11 kW single-phase or 17 kW three-phase) on the UK distribution network requires G99 connection approval before commissioning. For warehouse solar systems (typically 100 kW to 5 MW), G99 is the longest single critical path in the project timeline — typically 8-14 months on unconstrained networks, 14-24 months on constrained networks. This guide explains the G99 process, regional DNO timescales, typical reinforcement costs, and how we manage applications to minimise delay.
What is G99 and when is it required
G99 (Engineering Recommendation G99) is the technical standard that governs how embedded generation connects to Great Britain's distribution network. Replacing G59 in 2019, G99 covers all commercial-scale solar PV, wind, CHP and battery installations connected at the LV (low voltage) or 11/33 kV HV (high voltage) levels. G99 applies whenever total installed capacity at a single site exceeds 16 amps per phase (approximately 11 kW single-phase or 17 kW three-phase). For warehouse solar systems, G99 applies to essentially every installation above the smallest microgeneration size — 100 kW, 250 kW, 500 kW, 1 MW, 2 MW, 3 MW+ systems all require G99 approval. Systems above 1 MW also trigger additional protection studies and may require connection at 11 kV or 33 kV rather than 400 V LV.
The G99 application process — stage by stage
Stage 1: Application preparation (2-3 weeks). We prepare the full G99 application package including site location plan, single-line electrical diagram, inverter G99 type-test certificates, protection settings, anti-islanding configuration, and proposed metering arrangement. Stage 2: DNO acknowledgement and acceptance (1-2 weeks). The DNO confirms application is complete and queues for technical study. Stage 3: DNO technical study (8-16 weeks). The DNO assesses local network capacity, fault levels, voltage management, protection coordination, and any required reinforcement works. Stage 4: Connection offer (1-2 weeks after study). DNO issues formal connection offer with cost breakdown, connection date, and any conditions. Stage 5: Acceptance and works (variable). Customer accepts offer and pays connection charges. Reinforcement works (where required) are scheduled by DNO. Stage 6: Connection and commissioning (4-8 weeks). Final on-site connection works, inspection, witness testing, and energisation.
G99 timescales by UK DNO region (2026)
UK Power Networks (UKPN — London, East of England, South East): connection offer 13-18 weeks; reinforcement on constrained London network can add 6-12 months. Western Power Distribution / National Grid Electricity Distribution (WPD/NGED — Midlands, South West, South Wales): connection offer 14-20 weeks; reasonable network capacity outside major cities. Scottish & Southern Electricity Networks (SSEN — Scotland and Central/South England): connection offer 12-16 weeks; rural Scotland and Central Belt timelines can extend. Northern Powergrid (NPG — North East and Yorkshire): connection offer 12-15 weeks; strong network capacity. Electricity North West (ENW — Greater Manchester and North West): connection offer 14-20 weeks; central Manchester has been constrained but improving. SP Energy Networks (SPEN — Central Scotland and Merseyside/Cheshire): connection offer 13-17 weeks. Connection offer timescales lengthened across all DNOs during 2021-2024 due to rapid renewables growth; 2025-2026 has seen modest improvements as DNOs invest in network reinforcement.
Typical G99 connection costs by system size and region
G99 costs vary by system size, voltage level, and required reinforcement. For warehouse solar (LV connections, 100 kW-1 MW): typical connection cost £8,000-£35,000 with no reinforcement; £30,000-£150,000+ if reinforcement is required (transformer upgrade, cable replacement, or new switchgear). For larger systems (1-5 MW at LV or HV): typical connection cost £30,000-£250,000 without reinforcement; £150,000-£800,000+ with reinforcement. DNO reinforcement costs are now subject to "general reinforcement" cost sharing under Ofgem's Targeted Charging Review — customers pay only for site-specific costs, not network upgrades that benefit other users. We assess likely connection cost during initial feasibility based on grid heatmaps and historic DNO data for the area — providing an indicative cost before any commitment.
How we manage G99 applications to minimise delay
Five practical strategies to compress the G99 critical path. (1) Pre-application engagement: for systems above 1 MW we hold pre-application meetings with DNO engineers — early dialogue typically saves 4-8 weeks on application acceptance. (2) Complete application package: 70%+ of G99 applications go back for additional information at first submission, adding 2-4 weeks. We submit complete on first attempt. (3) Parallel workstreams: structural survey, electrical design, planning consultation and DNO application all run in parallel from week 1. (4) Quick acceptance: we monitor connection offer issuance and respond to acceptance within 5 working days (avoiding the DNO 90-day acceptance window expiring). (5) Customer advocacy: where reinforcement is proposed, we challenge cost allocation and timing — often able to reduce site contributions by 20-40% through detailed engineering review.
When G99 reinforcement is required — and how to assess upfront
Reinforcement is triggered when proposed generation exceeds the existing network capacity at the point of connection. Common triggers include: thermal limits on the local feeder cable being exceeded under reverse power flow conditions; voltage rise above the +6% statutory limit at remote points on the feeder; fault level at the substation exceeding switchgear ratings; insufficient transformer capacity at the primary substation; protection coordination conflicts with existing equipment. Each DNO publishes online Grid Capacity heatmaps (UKPN Connections Map, WPD/NGED ConnectMore, SSEN Grid Connections Insights, etc.) showing capacity availability by area. We use these tools plus historic application data to provide indicative reinforcement risk during feasibility. For high-risk sites, we recommend reducing system size, splitting across multiple connections, or applying for export limitation (G99 with active power curtailment) to avoid reinforcement.
G99 versus G98, G100, and special connection categories
G98: applies to microgeneration up to 16 amps per phase (about 11 kW single-phase) — installer notifies DNO after install, no pre-approval required. G99: applies to commercial generation above 16 amps per phase as covered on this page. G100: applies to large generators above 50 MW or connections at 132 kV+ — substantially more complex process involving National Grid ESO. Special categories include: G99 with export limitation (active power curtailment to grid limit — allows larger systems than the connection would otherwise accept); G99 fast-track applications (under £25,000 connection cost, sub-50 kW systems, can be expedited via Connect Direct-style portals); G99 alteration applications (for existing systems being uprated). For warehouse solar above 1 MW we typically need G99 with formal connection study; for sub-1 MW systems we may qualify for streamlined assessment depending on DNO and network conditions.
Common questions about g99 application
What is a G99 application?
A G99 application is the formal request to the local Distribution Network Operator (DNO) for permission to connect a commercial-scale generation system (solar PV, wind, battery) to the UK distribution network. G99 is the Engineering Recommendation that governs the technical standards. The application is required before any commercial solar system above 16 amps per phase (about 11 kW single-phase, 17 kW three-phase) can be commissioned and energised.
How long does a G99 application take?
End-to-end G99 timescale is typically 8-14 months on unconstrained networks. The DNO issues a connection offer 12-20 weeks after application depending on region. If reinforcement is required (transformer upgrade, cable replacement), this can add 6-18 months. We manage the entire process and provide fortnightly progress updates from submission through energisation.
How much does a G99 connection cost?
For typical warehouse solar (100 kW-1 MW LV connection): £8,000-£35,000 without reinforcement; £30,000-£150,000+ if reinforcement is required. For larger systems (1-5 MW): £30,000-£250,000 without reinforcement; £150,000-£800,000+ with reinforcement. We provide indicative connection cost during feasibility using DNO grid heatmaps and historic application data.
Which DNO covers my warehouse?
There are six UK DNOs operating distinct regional licensed areas: UKPN (London, East, South East), WPD/NGED (Midlands, South West, South Wales), SSEN (Scotland, parts of Central/South England), Northern Powergrid (North East, Yorkshire), Electricity North West (Greater Manchester, North West), and SP Energy Networks (Central Scotland, Merseyside, Cheshire). Each DNO has its own application portal, technical contacts, and processing timescale. We hold relationships with engineering teams at all six DNOs.
Can I install warehouse solar without G99 approval?
No. Installing and operating embedded generation above 16 amps per phase without DNO approval is a breach of the Distribution Code and the Electricity Safety, Quality and Continuity Regulations 2002 — potentially attracting enforcement action and disconnection. Insurance and warranty cover will also be void. All MCS-accredited installers will refuse to commission a system without G99 approval. The G99 process exists to protect public safety, network reliability and other connected customers.
Can G99 be done in parallel with construction?
Yes — and we always run G99 in parallel with structural survey, electrical design, planning and procurement. G99 typically remains the critical path so we submit immediately after structural feasibility (week 2-3). Construction and on-site installation can complete before G99 acceptance — the system is commissioned but cannot be energised until DNO connection works are complete. Where the DNO timescale exceeds 12 months, we discuss phased commissioning approaches with the customer in advance.