Commercial solar installation — what gets done, by whom, in what order.

The eight phases of UK commercial solar installation

Phase 1 — Site survey & structural assessment (week 1-2)

Engineers visit the site to confirm roof condition, structural capacity (PV adds 12–15 kg/m² to the roof load), shading, orientation, and electrical infrastructure. Pre-2000 portal-frame buildings often need additional purlin or bracing — we get a structural engineer's report at this stage. Output: detailed site survey report, structural sign-off, design constraints map.

Phase 2 — Electrical design & DNO application (week 2-4)

The electrical design specifies inverter sizing, AC infrastructure, switchgear, isolation points, and the connection method. The G99 application is then submitted to the local DNO (UK Power Networks, NGED, Northern Powergrid, SP Energy Networks, ENWL, or SSEN — depending on region). G99 turnaround is 60-110 working days. This is the long pole of the project — submitted week 4 latest, ideally week 1.

Phase 3 — MCS-compliant system design (week 3-5, parallel to Phase 2)

System design is finalised under MCS commercial standards. PVsyst modelling produces the yield assessment. Panel and inverter selection — Tier 1 modules (Trina, JA Solar, Jinko, Longi) and commercial-grade inverters (SMA, Huawei, Sungrow, Solis). Mounting structure (Schletter, K2, Van der Valk for rooftop; ground-mount for land arrays).

Phase 4 — Procurement & supplier contracts (week 5-7)

Supplier orders placed for panels, inverters, mounting, DC cabling, AC switchgear. Major lead times: inverters (2-6 weeks for commercial sizes), specialised mounting systems (3-5 weeks). For PSDS or Local Growth Fund projects, procurement runs through public-sector frameworks. For private projects, direct supplier engagement.

Phase 5 — DNO connection works (week 8-12, parallel)

If the DNO Connection Offer requires non-contestable network reinforcement (transformer upgrade, substation works, 11kV reinforcement), the DNO completes that work. Lead times for transformer upgrades have been 6-14 months globally in 2025-26 — confirm at G99 stage.

Phase 6 — Panel install & mounting (week 8-12)

On-site install. For sub-500kWp rooftop projects, typically 1-2 weeks of install time using IPAF-trained technicians and a small crew (2-4 installers). DC infrastructure is built up panel-by-panel as the array goes on. Health and safety follows CDM 2015 — site-specific RAMS.

Phase 7 — Inverter & AC connection (week 12-13)

Inverters mounted in plant room or external enclosure. AC switchgear installed and tested. Final commissioning electrical certification (NICEIC) completes the AC side. The DNO supplies the energisation date based on G99 schedule.

Phase 8 — Commissioning, MCS sign-off, M&V setup (week 13-16)

System energised, performance test against PVsyst yield model. MCS commercial certification issued. Insurance-backed warranty registered. Monitoring system (SolarEdge, Solar-Log, Huawei Smart PV, Sungrow iSolarCloud) set up with daily / monthly reporting. Handover documentation pack completed. Annual O&M plan in place.

Who actually does the installation

We don't install ourselves. We project-manage the build through one of six MCS-certified delivery partners across the UK:

• South East & London — partner installer covering UKPN territory
• South West & Wales — partner covering NGED Wales/SW
• Midlands — partner covering NGED Midlands
• North West — partner covering ENWL territory
• North East & Yorkshire — partner covering NPG territory
• Scotland & Northern Ireland — partners covering SP Energy Networks and NIE Networks territories

Each partner is an MCS-certified commercial installer with NICEIC or equivalent electrical certification. We hold one contract on behalf of the client; the delivery partner does the install. This keeps procurement clean and gives us 6+ years of relationship-based delivery quality data per partner.

What you should expect from your installer

Five things to confirm at quote stage:

1. MCS commercial certification — non-negotiable for grant routes and SEG eligibility
2. Insurance-backed warranty (IBW) — separately listed on quote
3. G99 DNO confirmation — pre-application response in writing before contracts
4. Itemised cost breakdown — panels + inverters together should be 50-55% of total quote
5. 0% VAT applied — explicit ask, some installers default to charging 20%