What Solar PV Installers Get Wrong on Their First Commercial Contract

UK solar PV capacity passed 20 GW by the end of 2025, a milestone the industry described as demonstrating solar's shift from a niche technology to a core pillar of the energy system. Commercial rooftop installations are a growing share of that total, and many solar businesses that built their model around residential work are now pursuing commercial contracts - drawn by larger system sizes and stronger order values. The problem is that a commercial solar project is not a scaled-up version of a residential one. The regulatory process, the procurement timing, the billing structure, and the documentation requirements all work differently. The assumptions that keep a residential installer profitable can cost a commercial installer months of delay and thousands of pounds in untracked costs.

The G99 Application That Should Have Gone In Earlier

Residential solar installations under 3.68 kW per phase operate under G98, which means notifying the Distribution Network Operator (DNO) after installation - no prior approval required. That keeps timelines predictable. Most commercial solar installations above 17 kW require G99 prior approval before the system can connect to the grid, and the approval process has lengthened significantly as renewable connection demand has outpaced network capacity across the UK.

The most common timing mistake is waiting until full system design is complete before submitting the G99 application. Specialist installers working across multiple commercial sites now submit G99 as soon as the structural survey is complete - typically within three to four weeks of contract signature. The design can be updated as it firms up; what cannot be changed is your position in the DNO's queue. Waiting until full design completion needlessly loses eight to twelve weeks of clock time.

On rural feeders and capacity-constrained networks - parts of Cornwall, West Wales, Yorkshire Dales feeders, and Highland and Islands areas - technical study responses alone can take 65 to 90 working days. Actual connection on heavily constrained networks can take six to fourteen months beyond that. The physical installation of a commercial rooftop system takes three to seven working days once approved. The DNO process is the binding constraint on overall project delivery, and managing it early is one of the clearest ways to protect your programme.

Ordering Equipment Into an Uncertain Window

Once the G99 application is submitted, the project enters a period of weeks or months where no physical work can proceed. Commercial installers face a procurement timing problem that residential work never presents: order equipment too early and it sits in storage, occupying warehouse space and carrying damage risk; order too late and lead times mean the equipment is not ready when the install window finally opens.

Commercial systems require tighter procurement coordination than residential ones. A rooftop commercial install typically involves larger panel arrays, three-phase inverters, larger cabling runs, and structural mounting systems sized to a specific structural survey - none of which can simply be substituted from stock if an order comes in late or the wrong specification is delivered.

The right approach is to place confirmed orders only after the DNO has issued a connection offer. At that point the install window is known, structural confirmation is in hand, and the equipment specification is final. Placing full equipment orders before the connection offer arrives creates unnecessary risk - DNO conditions can require design changes that make equipment already ordered unsuitable.

Stage Payments Don't Write Themselves Into the Contract

A residential solar project typically runs two to four weeks from survey to MCS sign-off. A commercial project often runs three to twelve months or more. On residential work, a deposit plus final payment keeps cash flow manageable. On commercial work, carrying equipment procurement costs and subcontractor payments across a months-long project without structured stage billing is a serious cash flow problem.

The mistake is not asking for stage payments - most commercial customers expect milestone billing. The mistake is not defining those milestones in writing before the contract is signed. Getting a customer to agree a payment schedule mid-project, after they have already committed to the order, is much harder than having them sign a milestone schedule at the outset.

A workable stage payment structure for commercial solar might look like: a deposit of 10 to 20% at contract signature, a further 20 to 30% on receipt of DNO approval, 20 to 30% on equipment delivery to site, 15 to 20% on installation completion, and the balance on commissioning and MCS sign-off. The exact splits depend on procurement exposure and project duration, but the milestones should be contractually defined before the first invoice is raised.

What Residential Job Costing Misses on Commercial Work

Residential solar job costing is relatively simple: panels, inverter, labor, scaffolding, and MCS certification. Commercial job costing covers a wider range of cost categories - and the ones most often missed at the quote stage are the ones that erode margin silently through the project.

DNO application and technical study fees are a real cost that varies by project size and network condition. On a 100 to 250 kWp commercial system, DNO connection charges can run from £4,000 to £25,000 or more, depending on the network works required. Structural certification, CDM compliance documentation (required for commercial projects above certain thresholds), electrical design fees, as-built drawings, and commissioning reports all carry time and cost that should be allocated to the job record at quote stage.

Subcontractor costs are another common gap. Commercial installs regularly bring in specialist electrical subcontractors for three-phase switchgear, inverter commissioning, or metering installation. Those costs need to be captured against the job, not absorbed into overhead. When they are not tracked at job level, the project shows a healthy margin at quote and a disappointing one at final account.

The businesses that struggle are those that apply residential job costing to a commercial project and spend months unpicking the consequences at final account.

The fix is building a commercial cost template before the first commercial quote goes out - one that includes every cost category specific to commercial work, from DNO fees to CDM administration, so nothing gets missed when the numbers are assembled.

Commercial solar is a genuine growth area for residential installers who are ready to scale. The businesses that make it work are operationally prepared before they sign the first commercial contract: they understand the DNO process, they have structured billing around project milestones, and they are tracking every cost from the start. The businesses that find commercial work harder than expected are usually running a residential workflow on a project that demands something different.

• DNO grid connection for rural UK farm solar in 2026Solar Panels for Farm Buildings · accessed 2026-06-24
• UK Solar: 2025 milestones and the outlook for 2026Solar Power Portal · accessed 2026-06-24
• The Complete Guide to DNO Applications for Commercial and Renewable Projects (UK)G59 Projects Ltd · accessed 2026-06-24

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