Two Commissioning Dates on Every Commercial Solar Project - and Only One of Them Is Yours

Every commercial solar installation has two commissioning dates. The first is the one your crew controls: panels fitted, cabling terminated, inverter configured, protection settings confirmed, and the installation tested to the point your engineers are satisfied. The second is the one the Distribution Network Operator controls: G99 application processed, connection approved, export enabled. Most commercial solar installers build their project programmes around the first date. Most of their payment problems come from the second.

What the G99 Process Actually Requires

A G99 application is a formal request to your local DNO for permission to connect and operate a generating system above 16 amps per phase - which covers virtually every commercial solar PV installation above 11 kW on a single-phase connection or 17 kW on three-phase. The application itself is not technically complex, but the timeline is rarely quick. For a commercial system up to 250kWp, the full G99 process typically takes 8 to 16 weeks end-to-end. Sites with constrained network capacity - rural feeders or congested industrial estates in certain regions - can extend to 26 weeks. Where the DNO determines that network reinforcement is required, the total process can run to six to twelve months.

The hidden delay within that timeline is resubmission. More than 70% of G99 applications are returned for additional information at first submission, adding two to four weeks before the DNO's technical review even begins. Common gaps include missing protection relay settings, incomplete inverter datasheets, and earthing arrangement details not provided. Each is a preparation issue, not a site issue, and each is avoidable.

The physical installation - panels, racking, cabling, inverter, protection systems - can all complete before G99 approval arrives. But the system cannot be energised until the DNO has issued its connection approval and, for systems above 50kWp, attended commissioning witness testing. For larger commercial and industrial installations, the G99 application has been described by specialist contractors as the longest single critical path in the project timeline. For smaller business-scale installations, the installer frequently treats it as background admin while the site team presses forward. That mismatch is where the payment problems begin.

Where the Invoice Gets Stuck

The standard commercial solar contract defines final payment as due on commissioning. The customer reads commissioning as "working system, exporting to the grid." The installer means "physical installation complete, tested and handed over." When the G99 application runs six weeks beyond the timeline discussed at sale - which is common, not exceptional - the customer withholds final payment on the basis that the system is not commissioned yet in the sense that matters to them.

The installer has already incurred all of its costs: equipment, labour, sub-contractors, any scaffolding or specialist access. The materials alone on a typical 100kWp to 500kWp commercial installation represent a significant portion of the contract value. Waiting six to twelve additional weeks to collect final payment after the physical work is complete creates a cash gap that, across multiple projects running simultaneously, compounds quickly. For context on how that gap behaves as a business grows, the insight on the cash gap that opens when business picks up addresses the same pattern across project-based businesses.

The problem is not the DNO's timeline. That timeline is known in outline before the contract is signed. The problem is that the contract, the payment schedule, and the customer conversation were all written as though there is one commissioning event rather than two.

Structuring Payments Around Milestones You Control

The practical fix is to separate the payment schedule from the DNO's timeline at contract signing, so that each payment falls against a milestone the installer controls directly.

A well-structured commercial solar payment schedule typically includes a deposit on contract signing (covering equipment procurement lead times and DNO application fees), a stage payment on delivery of materials to site, a stage payment on completion of physical installation and internal commissioning tests, and a small retention on grid connection and export activation. The guide to quoting and running a commercial solar PV installation covers how those stages sit within the full project workflow.

The critical variable is how much of the contract value sits on that final payment. If 30% to 40% is held until export activation, the installer has handed significant leverage to the DNO's timeline. Experienced commercial solar contractors keep their final payment at 5% to 10% of the contract sum - enough to ensure the customer remains engaged through grid connection, but not enough to create material cash exposure while the application sits in the DNO's queue.

The same logic applies to how jobs are tracked internally. Physical completion and grid connection are two distinct events with different owners. When both are tracked as separate project milestones with their own dates and associated payment triggers - rather than collapsed into a single "commissioned" status - the outstanding retention stays visible to whoever manages the job, and it does not fall off the radar while the DNO works through its process. Zigaflow's project tracking feature is built to handle exactly this kind of two-stage milestone structure across a concurrent portfolio of jobs.

Submitting on Day One - and Telling the Customer Why

The operational lever available on every commercial solar project is application timing. The day a customer signs the contract is the latest a G99 application should be submitted - not the day physical installation begins, and not the day it completes. Running the DNO application in parallel with structural survey, electrical design, and equipment procurement does not change the DNO's clock, but it prevents that clock from starting after the site work is already finished.

For systems above 1 MW, pre-application engagement with DNO engineering teams - a formal step that many installers skip - typically saves four to eight weeks on application acceptance. For smaller commercial systems, a complete and accurate first submission avoids the resubmission delay that catches more than 70% of first-time applications.

Commercial solar customers generally understand that equipment has lead times and that scaffolding needs advance booking. Fewer automatically understand that the grid connection is a separate regulatory process with its own queue, its own fees, and its own timeline. The installer who explains both commissioning stages clearly at contract signing - what the installation team will complete, and what the DNO then handles independently - rarely faces a disputed final invoice. The installer who skips that conversation usually does.

The cash exposure on commercial solar accumulates during the G99 wait, but the root cause is almost always a contract or communication gap set up weeks or months earlier. Keeping final payment tied to installer-controlled deliverables, submitting the G99 application on the day the contract is signed, and tracking both commissioning stages as distinct project events turns an unpredictable wait into a managed and expected stage of every job.

Sources

• G99 Application UK 2026: Warehouse Solar DNO Connection GuideSolarPanelsForWarehouses.co.uk · accessed 2026-09-07
• DNO G99 Connection For Commercial Solar UK 2026SolarPanelGrantsForBusinesses.co.uk · accessed 2026-09-07

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