How to Install Commercial Solar Panels: The Process

Installing commercial solar panels runs through seven stages: feasibility from half-hourly consumption data, a structural survey to BS EN 1991, system design, planning (usually Class J permitted development with 56-day prior approval), a G99 grid connection application to your DNO, the physical install, then commissioning and handover. End to end it typically takes six to nine months, and the G99 grid connection — not the roof work — is almost always the long pole. The install itself is the short part: a 100kWp rooftop system is often on the roof and producing within one to three weeks. Most of the timeline is the paperwork and the network operator queue that sit in front of it.

Here is what actually happens at each stage, what it costs you in time, and where projects stall.

Stage 1: Feasibility from half-hourly data

The first thing a competent installer asks for is your half-hourly (HH) consumption data — the meter readings your supplier records every 30 minutes. This is the single most important input, because the return on commercial solar is driven by self-consumption, not by how much you generate. A system sized to your daytime load earns ~24–28p/kWh in avoided import; every unit you export instead earns only the SEG rate of roughly 12–16p. Sizing from roof area alone gets this wrong in both directions.

With twelve months of HH data, a designer can model how much of each kWh generated you would actually use on site, how a battery would change that, and what payback looks like. As a rule of thumb commercial roofs need around 5–8 m² per kWp, but that is a sanity check on whether the array fits — the size itself comes from your load profile. Expect payback in the 4–8 year range, tightening to 3–5 years on high daytime-load sites such as manufacturing or cold storage.

Feasibility usually takes one to three weeks and should end with an indicative design, a generation and self-consumption estimate, and a budget cost. At ~£700–1,100/kWp ex-VAT, a 100kWp system lands around £82,000–£110,000; a 250kWp system around £150,000–£240,000. Solar has carried 0% VAT since April 2022.

Stage 2: Structural survey to BS EN 1991

Before anything is fixed to the roof, a structural engineer must confirm the building can carry the load — this is a precondition, not an optional extra. The relevant standard is BS EN 1991 (the Eurocode covering imposed, wind and snow loads). A pitched roof gains the dead weight of the modules and mounting; a flat roof typically uses a ballasted system that adds more weight but penetrates nothing, preserving the membrane warranty.

The survey checks the roof structure, covering condition and remaining service life. There is no point installing a 25–30 year array on a membrane with ten years left — re-roof first or the panels come off and go back on at your cost. The structural report either clears the design or sends it back to be lightened or re-zoned. Allow one to three weeks and budget for remedial work if the survey flags it.

Stage 3: Detailed design

With load data and a structural sign-off, the design is finalised: module layout, string configuration, inverter selection and sizing, cable routes, isolation and the connection point. Commercial modules are typically ~2m x 1.1m, 400–590W each, at 20–22% efficiency, degrading around 0.4–0.5% a year with 25–30 year performance warranties. The inverter is the part you will replace once, usually at 10–15 years — design for easy access to it.

This stage also sets whether you are adding battery storage now or leaving space for it. Batteries run ~£400–800/kWh and lift self-consumption from 30–50% (solar only) to 60–95%, which is where most of the extra return comes from. If grid services revenue is part of the case, that needs a route to market — it is not automatic. Design typically takes two to four weeks and produces the drawings the planning and grid applications are built on.

Stage 4: Planning — usually Class J prior approval

Most commercial rooftop solar in England now proceeds under permitted development. The 1 MW cap on commercial rooftop installations was removed in December 2023, so larger arrays fall under Class J permitted development with a 56-day prior approval process rather than a full planning application. Car-park solar canopies sit under Class OA. This is England-specific; check the position in Wales and Scotland.

Prior approval is faster and cheaper than full planning, but it is not instant — the local authority has 56 days, and listed buildings, conservation areas or unusual sites can still need a full application. Start this in parallel with the grid application, not after it. Allow up to two months and treat anything the council asks for promptly so the clock does not reset.

Stage 5: G99 grid connection — the long pole

This is where projects wait. Any system above roughly 50kW needs a G99 connection application to your Distribution Network Operator (DNO), and the DNO’s response time is the single biggest variable in the whole programme. Smaller systems may qualify for the simpler G98 notification, but most commercial arrays are G99.

The DNO assesses whether the local network can absorb your generation and export. The answer is sometimes a straightforward approval, sometimes an offer with conditions, and sometimes a quote for reinforcement works that materially changes the business case. Because the queue and assessment can take several months, you submit the G99 application as early as the design allows — ideally alongside planning — rather than waiting for everything else to be ready. If you take one thing from this article: get the G99 in early. It is the reason commercial solar runs six to nine months rather than six to nine weeks. Our planning and grid guide goes into the DNO process in more detail.

Stage 6: Installation

Once design, planning and the grid connection are cleared, the physical install is quick and largely undramatic. Scaffolding or roof access goes up, mounting is fixed (penetrating on pitched roofs, ballasted on flat), modules are placed and strung, DC and AC cabling is run, and the inverter and isolation are installed. A typical commercial rooftop system is installed in one to three weeks depending on size, roof complexity and weather.

The disruption to your operation is usually modest — most of the work is on the roof and in a plant room, not through your premises. A short, planned shutdown is needed at the connection point to tie in safely, which is scheduled around your operations. This is the stage people picture when they think about commercial solar installation, but by now most of the risk is already behind you.

Stage 7: Commissioning and handover

Commissioning is the final electrical sign-off: the installer tests every string, verifies the inverter and protection settings, confirms metering, and energises the system in line with the DNO’s G99 connection agreement. You should not be exporting or running the array until this is signed off.

Handover gives you the documentation that matters later — electrical certificates, the G99 commissioning confirmation, panel and inverter warranties, the operations and maintenance schedule, and access to the monitoring platform so you can see generation and self-consumption from day one. Insist on monitoring; it is how you catch a failed string or a drifting inverter before it costs you a season of generation. From here the system is in service and the only ongoing requirement is light maintenance — cleaning, inverter checks and the one mid-life inverter replacement.

Timeline summary and tax footnote

In a clean run, feasibility through commissioning takes six to nine months, with the G99 connection the variable that decides whether you are at the fast or slow end. Stack the work in parallel — submit planning and the G99 application as soon as the design supports them, rather than running the stages strictly one after another, and you compress the calendar considerably.

On tax, solar is a special-rate asset and qualifies for 100% relief through the Annual Investment Allowance (up to £1m), not the headline full-expensing regime — that gives only 50% first-year allowance on solar, applies to companies only, and is unavailable for assets bought to lease, so landlords typically use the AIA plus 6% writing-down allowance. Capital allowances reduce your taxable profit rather than handing back the cost pound for pound, and business rates are exempt to 2035. Always take professional advice on your own position before relying on any of this.

Ready to start at stage one? The fastest way to a realistic timeline and cost is to send us your half-hourly data — get a commercial solar quote and we will model feasibility, flag the structural and grid questions specific to your site, and tell you honestly where the long pole sits.