A commercial solar project in 2026 is a six-phase programme stretching anywhere from 8 weeks for a sub-100 kW office rooftop to 12 months for a megawatt-scale industrial site. Most of the variance is not the installer, the panels or the inverter — it is the DNO connection process and, where applicable, the structural and asbestos issues uncovered at survey. This page lays out the phases, the realistic 2026 durations for each, and how a competent installer compresses the felt timeline by running phases in parallel.
The six phases at a glance
Every commercial solar project — under 50 kW or above 1 MW — moves through the same six phases. The durations stretch and compress with system size, DNO region, building condition and customer decision speed, but the sequence is identical.
- Phase 1 (Week 0-1): Enquiry and desk feasibility. Initial contact, half-hourly meter data review, roof drawing review, postcode DNO check, indicative quote.
- Phase 2 (Week 2-4): Quote, survey, design freeze. On-site survey, structural assessment, electrical assessment, fixed-price proposal, contract sign.
- Phase 3 (Week 4-12 sub-100 kW, 4-32+ above): DNO application and procurement. G98 connect-and-notify or G99 application, equipment ordering, scaffolding planning.
- Phase 4 (1-4 weeks on site): Physical install. Scaffolding or MEWP, mounting system, panel layout, DC and AC cabling, inverter installation, switchgear tie-in.
- Phase 5 (Week N+1): Commissioning, witness test, energisation. DC string tests, AC tests, inverter configuration, DNO witness test (G99 only), monitoring activation, MCS certificate, customer handover.
- Phase 6 (ongoing): Operations and maintenance. Annual visit, performance monitoring, warranty claims, cleaning where required, inverter replacement scheduling.
Phase 1: Enquiry and desk feasibility (Week 0-1)
The first week is desk-based. We need three pieces of information to give you a credible indicative quote: 12 months of half-hourly consumption data (your supplier provides this free, sometimes on request, sometimes from your portal), a roof plan or recent satellite image showing usable area, and the MPAN of the site so we can identify the DNO region and check for any obvious capacity constraints. With those three things we can model an array size, an indicative cost band, an annual generation estimate, an offset percentage against your consumption profile, and a rough payback. This goes back to you inside 5-7 working days as a written indicative quote, free.
The desk feasibility phase is also where we screen for show-stoppers. A pre-2000 building with corrugated asbestos cement roofing flags an asbestos remediation requirement that could add £15k-£60k and 4-8 weeks to the project. A site on a severely constrained DNO feeder might face £50k+ of network reinforcement charges or a 24-month wait. A 60 kW design proposal on a single-phase supply hits the per-phase G98 limit immediately and forces a three-phase upgrade conversation. We surface all of this at desk stage so the customer can decide before any survey fees are committed.
Phase 2: Quote, survey, design freeze (Week 2-4)
The on-site survey takes one day and produces three deliverables: a structural assessment of the roof (load capacity, condition, age, any required strengthening), an electrical assessment (existing switchgear, available capacity, point of connection, three-phase availability), and a shading analysis using a Solmetric SunEye or drone photogrammetry. Where roof access for survey is unsafe — fragile sheeting, high pitch, weather — we substitute drone survey plus internal-only inspection. See our dedicated commercial solar survey page for the full survey scope.
From the survey output we model the system in PVSyst, freezing panel layout, string design, inverter sizing, mounting choice, AC route and metering position. The output is a fixed-price proposal — every line item priced, no provisional sums. Customers normally take 1-2 weeks to review and sign. Larger organisations with multi-stakeholder sign-off (board, finance committee, parent company) realistically take 3-6 weeks here. Time at this stage is on the customer side, not ours.
Phase 3: DNO application and procurement (Week 4-12 sub-100 kW, 4-32+ above)
This is where projects diverge sharply by size. Under 100 kW falls under the G98 connect-and-notify regime — a paperwork process, no DNO pre-approval required, typically 4-8 weeks acknowledgement and clearance. See our G98 application page. Above 100 kW falls under G99, which is a formal DNO study with a 65-working-day statutory clock and routine end-to-end durations of 6-18 months depending on DNO and network constraint. See our G99 application page for the full breakdown.
Procurement runs in parallel with the DNO process. Tier 1 panels (JA Solar, JinkoSolar, LONGi, Trina) are typically 4-6 week lead time in 2026 — UK distributor warehouses run high stock. Tier 1 inverters (SolarEdge, SMA, Fronius, Huawei) run 6-10 weeks for commercial models, sometimes longer for SMA Sunny Tripower CORE2 in spec-heavy configurations. Mounting systems (Schletter, K2, Renusol) are usually inside 4 weeks. We order long-lead items at contract sign so the kit is on site for install start.
Phase 4: Physical install (1-4 weeks on site)
The physical install duration scales roughly with system size. A 30 kW office rooftop is 5-7 working days. A 100 kW warehouse is 2-3 weeks. A 500 kW factory roof is 4-6 weeks. A 1 MW industrial estate is 8-12 weeks. Multi-phase projects on very large rooftops can be sequenced over 16+ weeks if the customer prefers smaller daily disruption.
The install sequence on a typical project: scaffolding or MEWP setup (2-3 days), mounting system installation (1-2 weeks depending on roof type — see our mounting systems page), panel placement and DC cabling (1-2 weeks), inverter and AC switchgear installation (3-5 days), AC tie-in to existing distribution board (2-3 days), monitoring and metering installation (1-2 days), final clean-down and snag-list closure (2-3 days). All sequencing assumes weather access — wet weeks at height are non-productive and add real schedule risk in winter.
For G99 projects, we deliberately schedule the physical install to overlap the DNO study window. Mounting, DC cabling and inverter installation can all proceed before the DNO connection offer is finalised. Only AC tie-in to the network waits for the offer to be accepted and any DNO connection works to be complete.
Phase 5: Commissioning, witness test, energisation (Week N+1)
Commissioning is a full week of structured testing and documentation. DC string testing covers open-circuit voltage, short-circuit current and maximum power-point voltage on every string against design figures. AC commissioning covers RCD trip tests, earth bond verification, neutral integrity, voltage measurement at the point of connection, and meter polarity. The inverter is configured to G98 or G99 settings — voltage and frequency trip envelopes, anti-islanding, ROCOF and vector shift detection where required. Monitoring is activated and tested end-to-end. The MCS certificate is generated and the EICR (Electrical Installation Condition Report) is signed off by the qualified electrician.
For G99 projects, the DNO witness test sits inside this phase. A DNO engineer attends site to verify protection relay settings, watch the inverter trip and reconnect, and sign off the as-installed configuration against the original application drawings. A clean witness test is signed off the same day and the system is energised onto the network. A failed witness test typically forces a 4-8 week return visit and re-test — not a small delay, which is why we double-check protection settings before the engineer arrives. The full commissioning sequence is on our commissioning page.
Phase 6: Operations and maintenance (ongoing)
From energisation onwards the system enters its 25-year operating life. The first year is monitored intensively to confirm performance against the PVSyst model — any underperformance in year one is investigated immediately under workmanship warranty. From year two onwards a routine O&M contract handles annual visits, monitoring review, cleaning where the site warrants it, warranty claims management and inverter replacement budgeting. Our commercial solar maintenance contract page covers tier options and pricing. Smart Export Guarantee registration with your chosen supplier is the final post-energisation administrative step — typically inside 14 days.
Total realistic 2026 timelines by system size
- Under 100 kW (G98): 8-16 weeks end-to-end. Healthy site with no asbestos, structural or capacity issues lands at the 8-week end. Asbestos remediation, structural strengthening or DNO capacity issues stretch to 16+ weeks.
- 100-500 kW (G99 routine): 4-9 months end-to-end. The G99 study window dominates the timeline. On Northern Powergrid or rural SSEN we routinely complete inside 4 months. On UK Power Networks central London or urban NGED we plan for 7-9 months.
- 500 kW-2 MW (G99 with potential reinforcement): 6-12 months. Reinforcement works on the 11 kV network, where required, are programmed by the DNO and routinely run 4-8 months in their own right.
- Above 2 MW (G99 plus NESO assessment): 9-18 months. Cross-border into transmission-level impact assessment adds another regulatory layer and typically 4-6 months.
What can compress the felt timeline
Five tactics consistently shave weeks or months off the customer-experienced timeline. Provide meter data and roof drawings at first contact. Customers who supply this proactively get an indicative quote and survey booking in week one, not week three. Decide quickly. Multi-stakeholder approval cycles routinely add 4-8 weeks. A clear sign-off authority cuts dead time. Run physical install in parallel with DNO study. Mounting, DC cabling, inverter install can all happen during the G99 window. Specify DNO-friendly inverters. Inverters with current G98/G99 type-test certificates and lower fault current contributions clear DNO scrutiny faster. Accept ANM curtailment offers where modelling shows curtailment under 5%. Waiting another 6 months for full reinforcement to win 4% of annual generation is rarely the right call — the IRR maths goes against it almost every time.
Where projects routinely slip
From hundreds of completed projects the slip points are predictable. Asbestos discovery at survey on pre-2000 corrugated cement roofs adds 4-8 weeks for licensed remediation. Structural strengthening required by the engineer's report adds 2-4 weeks. DNO queue management on constrained networks can extend the G99 study window from 65 working days into 6+ months on UK Power Networks central London, urban NGED, parts of SSEN southern coast. Switchgear or transformer lead times on large industrial sites with HV connections can run 12-24 weeks. Customer-side delays on contract review, board sign-off, finance approval — usually 4-12 weeks of the total elapsed time on enterprise projects.
Useful authority links
The MCS certification standard governs installer competency and is documented at mcscertified.com. DNO connection processes and timeline performance are regulated by Ofgem at ofgem.gov.uk. UK government net zero strategy and decarbonisation timelines are at gov.uk Net Zero Strategy.
Related decision pages
If you are still scoping the project, our cost guide covers system pricing across all sizes and our are commercial solar panels worth it page covers the financial case. For grid-connection specifics see G98 (under 100 kW) or G99 (above 100 kW). The commercial solar survey page covers what we look at on site. Once installed, the commissioning and maintenance contract pages cover the post-install operating life. For grants funding into the project, see grants and funding. Sector-specific timelines and constraints are in our factories, warehouses, offices and retail sector pages. The hub for everything commercial solar is commercial solar PV.
Installation timeline — common questions
How long does a typical 50 kW commercial solar installation take from enquiry to switch-on?
A 50 kW rooftop install on a healthy site sits comfortably inside the G98 fast-track route. Realistic 2026 end-to-end timeline is 8-12 weeks: 1 week desk feasibility, 2-3 weeks survey and design freeze, 2-4 weeks DNO connect-and-notify plus procurement (these run in parallel), 1-2 weeks on site, then 1 week for commissioning and witness test sign-off. Add 2-4 weeks if structural strengthening or asbestos works are needed, or if the inverter has long lead time.
What is the longest single delay risk on a commercial solar timeline?
For under 100 kW projects the biggest delay risk is asbestos sheeting (pre-2000 industrial roofs) or structural strengthening, either of which can add 4-8 weeks. For above 100 kW projects, by far the biggest delay is the DNO G99 process — a routine connection on UK Power Networks or NGED can take 9-15 months, with 24+ months on severely constrained urban networks. We model this upfront so the customer is not surprised six months in.
Can the install run in parallel with the DNO application?
Yes — and this is where competent installers shave 8-16 weeks off the felt timeline. Panel mounting, DC cabling, inverter installation and even AC wiring inside the building can all happen during the DNO study window. The only thing that physically waits for the DNO is the final tie-in and witness test for energisation. We always programme physical works to overlap the DNO clock.
When is the best time of year to install commercial solar?
Spring (March-May) and autumn (September-October) are operationally easiest — long enough days, dry enough weather, manageable temperatures for working at height. December-February installs are slower because of weather access restrictions on roofs and short daylight. June-August is fine but installer diaries are usually busiest. The financial argument: every month of delay is a month of avoided generation, so the right answer is "the earliest the timeline allows" — not waiting for a notional better season.
How quickly can I get a quote and signed contract?
A fixed-price quote based on desk feasibility — half-hourly meter data, roof drawings, postcode-level DNO heat map check — takes 5-7 working days from enquiry. An on-site survey adds another week or two. Customers who supply meter data at first contact and have a sensible decision-making process can go from enquiry to signed contract in 3-4 weeks. Slower customers (multi-stakeholder approvals, board sign-off) realistically take 6-10 weeks at this stage alone.
What does the commissioning week actually involve?
Commissioning is a full week of work, not a day. It covers DC string testing (Vmpp, Voc, Isc on every string), AC checks (RCD, earth bond, voltage), inverter configuration to G98/G99 protection settings, monitoring setup and handover, customer training, MCS certificate issue, EICR (Electrical Installation Condition Report) and — for G99 — the DNO witness test. Our /solar-panel-commissioning/ page walks through every step.
Do I need to be on site during installation?
Not for the bulk of the work. We need site access, a single point of contact for keys and roof access, and a brief meeting at the start of week one (toolbox talk, RAMS sign-off, isolation procedure agreement). Day-to-day install we manage independently. We need the building open for AC tie-in and final commissioning — usually 2-3 days at the end of the programme. Customer staff training can be scheduled at any convenient hour during the final week.