G99 — formally Engineering Recommendation G99 Issue 1 Amendment 9 (2026) — is the standard UK distribution network operators (DNOs) use to govern the connection of generation above 17 kW per phase or 100 kW total to the low-voltage and 11 kV distribution network. For any commercial solar PV installation above 100 kW, your DNO must approve the connection before energisation, and the process can take anywhere from 6 months on an unconstrained rural network to 24 months on a severely constrained urban grid. This page lays out exactly what happens, what it costs, where projects get stuck, and how a competent installer compresses the timeline.
What G99 actually is — and where it differs from G98
G99 is one of two Engineering Recommendations published by the Energy Networks Association governing connection of distributed generation in the UK. Its smaller sibling, G98, covers connect-and-notify installations up to 17 kW per phase / 100 kW total — straightforward, fast, no pre-approval required. G99 covers everything above that threshold, and unlike G98 the DNO must approve the connection before any energisation. The application is a formal study by the DNO of how your proposed generation will affect their network: voltage rise, fault levels, thermal capacity of cables and transformers, and protection coordination. It is also where the DNO charges any required network reinforcement back to you.
Trigger points for G99 are unambiguous: above 17 kW per phase on any phase, or above 100 kW total across all phases. A 96 kW install on a three-phase 400 V supply (32 kW per phase, balanced) sits inside G98. A 110 kW install (37 kW per phase) is firmly G99. A 60 kW install on a single-phase 230 V supply is also G99 because it exceeds the per-phase limit — most small commercial sites with single-phase supplies hit this constraint and have to choose between a three-phase upgrade or a smaller G98-compliant array. We size every quote against this threshold deliberately, not by accident.
The seven stages of a G99 application
Every G99 application moves through the same sequence regardless of DNO. Knowing the stages lets you see where the project is and what is realistically the next bottleneck.
Stage 1: Eligibility and pre-application check
Before lodging anything, we run a desk-based check of your supply: import capacity (Available Supply Capacity on the MPAN), phasing, distance to the nearest substation, and known network constraints in the postcode. The DNO publishes a heat map (UK Power Networks' Network Capacity Map, NGED's Network Capacity Map, SSEN's Distribution Future Energy Scenarios) showing where the 11 kV network is constrained. Sites in red zones face longer timelines and higher reinforcement costs and we factor this into the quote upfront. This stage takes 2-5 working days and costs nothing — the data is open.
Stage 2: Application submission
The formal application is lodged on the DNO's connection portal with: customer details and MPAN, single line diagram showing the existing supply, proposed PV array configuration, inverter type-test certificates (PDF), protection settings, generator data sheet, expected energisation date, and a non-refundable application fee (typically £350-£500 for under-200 kW, scaling up to £1,500-£15,000+ for very large installs). The DNO has 5 working days to acknowledge and 65 working days to issue the connection offer — those clocks are statutory under the Connection and Use of System Code (CUSC).
Stage 3: DNO study and connection offer
This is the longest stage at this point in the process and what you are paying for. The DNO models the proposed generation against their power-flow analysis of the local network, looking at voltage rise, thermal capacity of the upstream transformer and 11 kV cables, fault levels, and protection coordination. They issue a written connection offer that says either "you can connect at the proposed capacity with no network reinforcement" (best case), or "you can connect with limited capacity / curtailment under ANM" (good case), or "you can connect but the network needs reinforcement at a cost of £X" (variable case). Expect 8-13 weeks for a routine study, longer where the network is constrained or where the application has gone into queue management.
Stage 4: Offer review and acceptance
You have 90 days from the date of offer to accept it or it lapses. Accepting is a binding commitment to pay the connection charges and proceed with the project. Before accepting we model the financial impact of any curtailment terms or reinforcement costs in the project DCF — sometimes a higher capacity offer with reinforcement is worse for IRR than a lower curtailed offer with no reinforcement. We share that comparison with the customer alongside the offer review.
Stage 5: Connection works and reinforcement
If the offer requires DNO works (cable reinforcement, transformer upgrade, new substation tie-in), they happen at this stage. The DNO programmes the work into their delivery calendar — this is where 6-month delays creep in on constrained networks because the queue of customer connection works runs deep. We chase weekly and escalate where DNO programme dates slip materially against the offer.
Stage 6: Physical install
Panel mounting, DC cabling, inverter installation, AC cabling and switchgear all run in this stage and largely run in parallel with Stages 4 and 5 — we don't wait for the DNO to be ready. The only thing we don't do until the DNO has finished is the final tie-in to the network and energisation.
Stage 7: Witness test and energisation
A DNO engineer attends site to verify protection relay settings, anti-islanding response, earthing, and as-installed configuration against the application. A clean witness test is signed off same-day and the system is formally energised onto the network. Generation begins immediately. SEG (Smart Export Guarantee) registration follows within 14 days post-energisation.
Typical timescales by DNO
UK distribution networks are split between six DNO licensees. Performance varies materially. Below are realistic 2026 ranges for a 200-500 kW PV connection assuming routine application without major reinforcement.
- UK Power Networks (London, East, South East): 9-15 months. Heavy queue, urban constraints in central London and parts of Hertfordshire. Strong portal and clear documentation. ANM available on most constrained feeders.
- Northern Powergrid (Yorkshire, North East): 6-10 months. Lower queue, more rural network with capacity headroom. Generally the fastest of the major DNOs for routine PV connections.
- SP Energy Networks (Central Scotland, Merseyside, North Wales): 9-13 months. Some constrained networks around Liverpool and Glasgow. ANM and flexible connections widely offered.
- SSEN (Southern, Scottish Hydro): 8-14 months. Wide variance — Scottish Highlands often 6 months, southern coast and Hampshire/Berkshire run 12+ months due to congestion.
- National Grid Electricity Distribution (formerly Western Power, covering South West, Midlands, South Wales): 9-16 months. Cornwall and Somerset are particularly constrained for solar export.
- NIE Networks (Northern Ireland): 7-12 months. Smaller network, separate regulatory framework but similar G99 process.
Above 1 MW the project may also need to cross into the National Energy System Operator (NESO, formerly National Grid ESO) approval process for transmission-level impact assessment — that adds another layer and another 6-12 months. Most SME and mid-market commercial PV projects stay inside the DNO process exclusively.
What the application costs
Three cost categories matter. Application study fee is the desk fee paid to the DNO when the application is lodged. Currently £350-£500 for sub-200 kW connections, £750-£3,000 for 200-1,000 kW, and £3,000-£15,000+ for over 1 MW. Network reinforcement charges are project-specific and can range from £0 (healthy network) to £250,000+ (full transformer upgrade). On constrained networks the DNO is required to offer a flexible connection (ANM curtailment) as an alternative to reinforcement — flexible connections almost always have lower upfront cost and we usually recommend them unless modelling shows curtailment >8% of annual generation. Our project administration fee covers the time to manage the entire process — included in the install quote, never a separate line item.
For comparable cost ranges across project sizes, see our full cost guide. Asset finance covers all of this — see finance options.
What the DNO needs from you
A complete G99 application bundle includes: customer details and active MPAN, current Available Supply Capacity, single line diagram showing the existing connection arrangement, proposed PV array drawing showing inverter location, generator data sheet, type test certificates for every inverter model on the system, protection coordination study, expected energisation date. Missing information sends the application back to "incomplete" status and stops the 65-day clock — a single missed protection setting can add 4-8 weeks to your timeline. We assemble everything in-house against a checklist before lodging.
Common pitfalls and how to avoid them
Capacity-constrained networks. If your site sits on a feeder already carrying multiple PV connections, you may face reinforcement charges even on a modest install. Check the DNO's network capacity heat map before designing the array — it sometimes pays to size to 95 kW (G98) rather than push to 110 kW G99 and trigger £40k of reinforcement.
Fault level limits. Older 11 kV substations can hit fault current limits when too much rotating or inverter generation is added — limits the DNO will protect by quoting reinforcement or by refusing capacity. Modern type-tested inverters with low-fault-contribution settings sometimes resolve this without reinforcement.
Single-phase site assumed three-phase. A small office originally on a 100 A single-phase supply can only host ~17 kW of PV under G98 / G99 phase-balance rules. Anything bigger forces a three-phase upgrade — typically £3,000-£15,000 — that should be costed into the proposal upfront, not bolted on as a change order.
ANM curtailment misunderstood. Some installers either ignore curtailment in DCF (overstating IRR) or panic about it (recommending against ANM offers that are actually fine). 1-5% curtailment is normal and acceptable; 10%+ is material. Always model.
Inverter type-test mismatch. Specifying inverters not on the current G99 type-test register fails the application instantly. We only specify currently certified inverters from established brands.
How we manage the process for clients
Every commercial solar quote above 100 kW we issue includes full G99 management as part of the headline price — no separate line item, no surprise admin fee. That covers: pre-application desk study including DNO heat map check; full application bundle assembly to DNO checklist; submission via the DNO portal and acknowledgement chase; ongoing liaison with the DNO connection engineer including weekly status during the study period; offer review with financial modelling of any curtailment or reinforcement terms; acceptance recommendation against the project DCF; scheduling of network works in coordination with our install programme; witness test attendance; energisation paperwork and SEG registration. Customers see fortnightly progress updates with explicit DNO milestone tracking. If the DNO programme slips materially we escalate through the DNO connection manager and, where warranted, to the regulator (Ofgem). See our about page for the team and accreditations we hold.
How to compress the timeline
Five tactics consistently shave weeks or months off the felt timeline. Lodge early. The 65-day study clock starts when the application is complete. Lodging while the customer is still finalising contract pushes the clock forward by weeks. Run physical install in parallel. Panel mounting, DC cabling and inverter install can run during the DNO study — only AC tie-in waits for the connection works. Accept ANM offers where curtailment is modelled below 5%. Waiting another 6 months for full reinforcement to win 4% of annual generation is rarely the right call. Specify DNO-friendly inverters. Some inverter models have lower fault current contributions and clear higher G99 thresholds without triggering reinforcement — the right model selection at design stage avoids the wrong conversation at study stage. Pre-empt single-phase / three-phase issues. A site that needs a three-phase upgrade is best identified at quote stage so the upgrade and the PV install run on coordinated programmes.
Useful authority links
The Energy Networks Association publishes the canonical G99 standard and updates: Energy Networks Association — Distributed Generation. Each DNO's connection portal and capacity heat map is the source of truth for your specific site. Ofgem regulates connection charging and timeline performance: Ofgem. The MCS certification framework governs installer competency for sub-50 kW domestic and small commercial PV: MCS. For broader UK government net zero policy context, see gov.uk Net Zero Strategy.
Related decision pages
If your project is below 100 kW, see G98 application process. To understand why solar still pays back even with a 12-month G99 process, read are commercial solar panels worth it. For larger industrial sites, the industrial solar panels hub covers HV and 11 kV connection. To compare solar against alternatives at the same capex, see solar vs alternatives. For sector-specific quotes see factories, warehouses, or cold storage. To pull funding into the project, our grants and funding page covers all current schemes.
G99 application — common questions
When does my commercial solar project need a G99 application instead of G98?
G99 applies whenever the proposed generation exceeds 17 kW per phase or 100 kW total across all phases. In practice, anything above 100 kW on a three-phase 400 V supply triggers G99. A single-phase site is forced into G99 territory at any installation above ~17 kW (about 60 panels) — which is why most genuine commercial sites with single-phase supplies need a three-phase upgrade before the PV install can proceed.
How long does a G99 application take in 2026?
Application acknowledgement is statutorily within 5 working days. The DNO connection offer must follow within 65 working days for a standard application (3 months). Acceptance and physical works then run another 3-12 months depending on whether network reinforcement is required. Realistic end-to-end timeline is 6-9 months for unconstrained networks (Northern Powergrid, SSEN rural, parts of NIE), 9-15 months for moderately constrained networks (UK Power Networks, SP Energy Networks, NGED Western), and 15-24+ months for severely constrained urban or southern networks. Plan accordingly.
How much does a G99 application cost?
The application study itself runs £1,500-£15,000+ depending on capacity and DNO. UK Power Networks runs higher than Northern Powergrid for comparable studies. Network reinforcement charges are the wild card: a connection on a healthy substation costs £0 in reinforcement; a constrained network might quote £30,000-£250,000+ to upgrade transformers, reinforce HV cables, or extend the local network. Curtailment-based connections (ANM and flexible connections) usually waive reinforcement at the cost of accepting export limits during peak network stress.
What is Active Network Management (ANM) and how does it affect my project?
ANM is a real-time control system DNOs use to throttle generators when the local network would otherwise hit thermal or fault-level limits. If your connection offer comes with ANM constraint, your inverter will be remotely curtailed (output capped) during peak network stress hours. In practice ANM curtailment averages 1-5% of annual generation on UK PV connections — far less than the cost and time of full network reinforcement. We always model curtailment impact in PVSyst before signing acceptance.
Can I install panels on the roof while waiting for the G99 connection offer?
Yes — the physical install can run in parallel with the DNO process up to (but not including) energisation and witness testing. We typically schedule structural work, panel mounting, DC cabling and inverter installation before the connection offer is finalised, then complete AC tie-in and DNO witness test once the offer is accepted and connection works are complete. This compresses the felt timeline for the customer by 8-16 weeks.
What inverters and protection settings do DNOs accept under G99?
Inverters must hold valid type-test certification under EREC G99 Issue 1 Amendment 9 (current 2026 standard) or later. Protection settings must include voltage and frequency relays meeting the specified trip envelopes, anti-islanding, ROCOF and vector shift detection where required. Any inverter brand we specify (SolarEdge, SMA, Fronius, Huawei, Sungrow, Solis) holds current G99 type certificates. Protection settings are configured per the connection offer — wrong settings fail witness testing and force a return visit.
What happens at the witness test?
A DNO engineer attends site to verify protection relay settings, anti-islanding behaviour, and overall installation compliance. They will trip the inverter, verify it disconnects within the required milliseconds, check earthing, and review the as-installed single line diagram against the design submitted at application. A clean witness test is signed off the same day and the system is energised onto the network. A failure means re-work, a return visit (usually 4-8 weeks later), and rescheduled commissioning — material delay we work hard to avoid.
Do you handle the G99 process or do I need to manage it?
We handle the entire process end-to-end as part of every commercial solar quote above 100 kW. That covers eligibility check, single line diagram, type test certificates, protection settings, application submission, DNO liaison, offer review and acceptance recommendation, scheduling of network works, witness test attendance, and energisation paperwork. You see status updates at each stage but the process administration sits with us. The customer cost is built into the install quote — no surprise add-ons.