Solar Panel Commissioning: Process & Standards

Commissioning is the most under-discussed phase of a commercial solar project — and the one that separates a properly working system from a sub-spec one. It is a full week of structured electrical testing, configuration and documentation that converts a physically installed array into a certified, energised, monitored, insured and SEG-registered generating asset. This page walks through every step: pre-commissioning checks, DC string testing, AC checks, inverter configuration, the DNO witness test for G99 connections, monitoring activation, MCS certification, EICR sign-off, customer training, insurance notification and SEG registration.

The seven commissioning steps

Every commercial solar commissioning programme moves through the same seven steps in sequence. Skipping any of them is the difference between a system that works for 25 years and one that fails inside year five.

Step 1: Pre-commissioning checks (1 day)

Before any energised testing, the commissioning engineer walks the entire installation against the as-built drawings. Visual inspection of every panel for transit damage, every string-cable termination for correct polarity, every clamp for torque, every isolator for correct labelling. Mechanical checks confirm the mounting system is fully secured to manufacturer torque specification (recorded in writing — important for the workmanship warranty), all cable clips are spaced correctly to prevent UV degradation, all roof penetrations (where present) are correctly sealed.

Pre-commissioning also verifies the AC switchgear isolation procedure works — the system can be isolated from the existing site electrical supply both at the inverter and at the point of connection. This matters for fire-safety compliance and for any future maintenance work on the building.

Step 2: DC string testing (1-2 days on 100 kW)

DC string testing is the most labour-intensive part of commissioning. Every individual string is tested for: open-circuit voltage (Voc) at the relevant cell temperature, short-circuit current (Isc), maximum power-point voltage (Vmpp) and current (Impp), insulation resistance (megger test), polarity confirmation. The measured values are checked against the manufacturer's data sheet figures corrected for the actual cell temperature at test time — a measured Voc that is more than 5% below predicted indicates a problem with the string (a damaged module, a poor termination, or a wiring fault).

DC string testing is done with calibrated test equipment (Seaward PV200, Megger PVM210, HT IV400) and the results are logged on a per-string basis. The string test report is part of the commissioning pack the customer receives at handover. On larger systems with module-level optimisers (SolarEdge, Tigo TS4) the testing extends to per-optimiser data and we use the cloud platform to verify every optimiser is reporting correctly.

Step 3: AC checks (1 day)

AC commissioning verifies the installation downstream of the inverter and back into the existing site. Tests include: continuity of protective conductors, insulation resistance of AC wiring, polarity of supply and earth, RCD trip times (must trip within 40 ms at rated tripping current), earth fault loop impedance, voltage at the point of connection (must be within DNO statutory limits — 230 V plus 10% / minus 6%), prospective short-circuit current at the point of connection. All AC checks are documented per BS 7671 (the IET Wiring Regulations) and form the basis of the EICR.

Step 4: Inverter configuration and energisation (1 day)

The inverter is configured against the specific G98 or G99 protection settings issued by the DNO for this site. Settings include: voltage trip envelope (over-voltage and under-voltage thresholds and time delays), frequency trip envelope (over-frequency and under-frequency thresholds and time delays), anti-islanding response, ROCOF (Rate of Change of Frequency) and vector shift detection where required, export limit (where the connection offer caps export), reactive power control settings.

Wrong settings fail witness testing on G99 projects and force a return visit — so we double-check every setting against the application paperwork before energising. On systems with multiple inverters, every inverter is configured identically and verified individually. The inverter is then energised, the array is brought online, and the system runs for several hours under live conditions to confirm steady-state operation.

Step 5: DNO witness test (G99 only, 1 day)

For G99 connections (above 100 kW or 17 kW per phase), a DNO engineer attends site to verify the system meets the connection offer requirements. The witness test typically covers: physical inspection of the as-installed configuration against the application drawings, verification of protection relay settings, observation of the inverter tripping under simulated voltage and frequency excursions, confirmation that anti-islanding response disconnects the inverter within the required milliseconds, earthing measurements.

A clean witness test is signed off by the DNO engineer the same day and the system is formally energised onto the network. A failed witness test forces a re-work and a return visit, typically 4-8 weeks later. The most common failure modes are wrong protection settings, missing documentation, or as-installed differences from the application drawings — all preventable with careful preparation. We treat the witness test as the highest-stakes day of the project.

Step 6: Monitoring activation and customer training (1 day)

The monitoring system is activated and tested end-to-end. Every Tier 1 inverter (SolarEdge, SMA, Fronius, Huawei, Solis, Sungrow, GoodWe) ships with a cloud monitoring platform — we connect the inverter to the customer's network or a dedicated 4G data SIM, register the system in the cloud platform, and verify that generation, consumption (where current sensors are fitted), export, voltage and inverter status data all flow correctly into the platform. The customer is given login credentials, a walkthrough of the dashboards, and a one-page reference card explaining typical operating ranges and alarm states.

Customer training is typically 90 minutes on-site at the end of commissioning week. We cover the monitoring platform, manual isolation procedure (for emergency only), routine visual checks the facilities team can perform, and the escalation path for any fault. We leave a laminated quick-reference at the inverter and a printed O&M manual on site.

Step 7: Documentation, MCS certificate, EICR, SEG registration (parallel through commissioning week)

Documentation runs in parallel with the technical commissioning steps. Deliverables to the customer include: full O&M manual (typically 80-150 pages depending on system size), DC and AC schematics as-installed, complete list of panel and inverter serial numbers (important for warranty claims), mounting system specification and torque records, electrical certs (Building Regulations Part P notification, EICR, RCD test record), MCS certificate (for systems within MCS scope, currently sub-1 MW), insurance notification pack, SEG registration submission to your chosen export supplier.

What the customer receives at handover

The handover pack is substantial — typically a 2-3 inch ring binder plus digital copies. Contents include the items listed above plus: monitoring login credentials, contact details for the workmanship warranty (us — typically 10 years IWA-backed), contact details for the panel and inverter manufacturers (for product warranty claims), the maintenance schedule and any associated O&M contract terms, a one-page emergency procedure card laminated at the inverter, and the post-energisation visit schedule for year-one performance verification.

Common commissioning failures and how to avoid them

Five issues account for most commissioning problems. Wrong protection settings. Inverter settings configured against the wrong DNO connection offer, or not updated when the offer was revised. We always cross-check settings against the latest paperwork. String wiring errors. A reversed-polarity string fails DC test immediately — the panel reads negative voltage. We trace and correct before the test is repeated. Earth bonding failures. A loose or missing earth bond fails AC tests and creates a real safety risk. We torque-test every bond at commissioning. RCD failures. An RCD that does not trip within spec is replaced and re-tested. Monitoring connectivity failures. Network or 4G coverage issues at the inverter location prevent cloud upload. We test connectivity at survey stage and specify the right comms before install.

Insurance notification

Your buildings insurer needs written notification of the new generation system. We provide the notification pack — system specification, mounting system type, panel and inverter serial numbers, MCS certificate, EICR, sign-off from the qualified electrician — at handover so the customer can forward to their insurer the same week. Most commercial insurers (Aviva, Allianz, AXA, Zurich and similar) accept MCS-certified solar PV installations without premium adjustment. A small minority add a modest premium reflecting the increased plant value on the roof. Failure to notify can void cover for any subsequent claim involving the roof — so we treat it as part of the handover discipline, not an optional extra.

SEG registration

Smart Export Guarantee registration converts the metered export into pence per kWh paid back to the customer. Current 2026 rates range 4-15 p/kWh depending on supplier and tariff structure. Registration requires the MCS certificate, MPAN, system specification and a recent meter reading. We submit the registration to the customer's chosen SEG supplier (most commonly Octopus Energy Outgoing or E.ON Next Export Plus) and the supplier confirms registration within 14-28 days. After that, export payments arrive monthly or quarterly depending on supplier.

Useful authority links

The MCS certification scheme details and database are at mcscertified.com. NICEIC governs electrical installer competency: niceic.com. The Smart Export Guarantee is regulated by Ofgem: Ofgem SEG. The IET Wiring Regulations BS 7671 govern AC commissioning standards: IET Electrical.

Related decision pages

For the project programme that gets you to commissioning see commercial solar installation timeline. For the DNO process feeding into the witness test see G98 application and G99 application. For the post-commissioning operating phase see commercial solar maintenance contract and our maintenance service. For the survey work that precedes commissioning see commercial solar survey. For inverter selection and configuration see best commercial solar inverters. For broader site sectors see factories, warehouses, cold storage. The hub for everything commercial solar is commercial solar PV. To start with a quote, go to request a quote.