Commercial PV installations, start to finish

The desk feasibility, before anyone sets foot on the roof

Nothing should be quoted from a roof photograph. The first stage is a desk exercise: usable roof area measured from imagery and site plans, with rooflights, plant, walkways and setbacks subtracted, then converted to capacity at roughly 5 to 6 m² per kWp. A 3,000 m² warehouse roof rarely yields 3,000 m² of array.

Your consumption profile is read alongside it. UK generation runs about 900 to 1,050 kWh per installed kWp per year, and a sensible design targets 60 to 85% of annual consumption rather than filling every metre of roof. Without storage, a daytime-occupied building typically uses 55 to 75% of what it generates.

What comes back is a size band, an indicative cost band and a view on the connection. It should sometimes come back negative. A shaded north-light roof over a unit that empties at four o'clock is a poor candidate, and saying so costs a day rather than a survey. If you are holding competing quotations, reduce each to a £ per kWp figure before comparing them, as the <a href="/cost/">commercial solar cost guide</a> explains.

Survey day: what a surveyor is measuring

A survey is not a walk round with a clipboard, and what gets missed on the day turns into a variation later.

The roof comes first: sheet profile and gauge, purlin spacing and span, fixing condition, existing penetrations, the state of laps and flashings, and how much life the covering has left. Then the electrical side. Intake position, main switchboard rating, spare ways, cable routes from roof to switchroom, and whether there is dry, ventilated space for inverters and AC isolation.

Shading is mapped rather than estimated: flues, plant, parapets, lift overruns, neighbouring buildings, boundary trees. Access is mapped too, because access drives cost. A yard with room for a telehandler is a different job from a city-centre unit where every pallet goes up one scaffold hoist.

Structure, asbestos and the roof warranty

Three checks decide whether a roof is buildable, and all three come before anything is ordered.

Roofs over about 1,000 m² usually warrant a structural appraisal by a chartered engineer, covering added distributed load, point loads at the fixings, and wind uplift across a modified roof surface. Older portal frames with light purlins sometimes need localised strengthening. Occasionally the outcome is a smaller array on the strongest bays, which is a better answer than an argument in week three.

Any building constructed before 2000 needs an asbestos survey before roof work starts, because cement sheet, gutter linings and older board still turn up on industrial roofs. Where the work may disturb the fabric, and roof fixing usually does, that means a refurbishment and demolition survey rather than a management survey. Finding asbestos does not stop the job. It changes the method, who is allowed on the roof, and the programme.

On profiled metal roofs, non-penetrative clip-fix mounting clamps to the seam or the sheet crown and puts no new holes in the covering, which is what preserves most roof warranties. On membrane and flat roofs the approach differs, and the manufacturer's position is worth having in writing beforehand. If the covering has only a few years left, re-roof first and mount second.

Detailed design and the connection application, running together

Detailed design turns feasibility into buildable drawings: roof layout with string numbering, a single-line diagram, string voltages checked across the full temperature range, cable sizing and volt drop, protection and isolation, and a fixing schedule tied to the structural appraisal. A fixed price is built on that package, not on a spreadsheet.

The connection application goes in as soon as the design is firm, because the network operator's answer can cap the sensible size of the system. Which route it takes turns on a threshold of roughly 50 kW, or 3.68 kW per phase.

Nobody on the contractor side controls that clock. What a competent installer does is submit early, submit correctly first time, and give you the network operator's date rather than a comfortable one.

What a working building experiences while the roof is live

Expect a fenced laydown area, typically a small block of parking bays. Expect one heavy delivery day and a single crane or telehandler lift to get pallets of frame and panels onto the roof. Expect edge protection or scaffold at the working area, a permit-to-work regime, and your own roof access rules followed rather than ignored.

Noise is modest, because clip-fix mounting is torque work rather than drilling. Internal disruption sits in the switchroom: connecting the AC side normally needs one or two short planned board shutdowns, booked into a shift gap, a night or a Sunday to suit the building.

Shift patterns, forklift traffic, trading hours, term dates. All of these are programme inputs, and any of them can be worked around when they are stated at survey rather than discovered in week two.

CDM 2015 and what competence looks like on a roof

Most commercial installations put more than one contractor on site, which brings the Construction (Design and Management) Regulations 2015 into play. They make whoever commissions the work the Client: the owner on an owner-occupied building, the tenant where a leaseholder is procuring the array.

If that is you, the duties are light but real. Appoint a principal designer and principal contractor in writing, allow sensible time rather than compressing the programme to a board date, and check that a construction phase plan exists before anyone starts. Most solar installations are not notifiable to the HSE: notification needs work lasting over 30 working days with more than 20 people on site at once, or exceeding 500 person-days.

Underneath the paperwork, competence is visible on site. Method statements written for your roof, not a template with the address changed. A fragile-roof assessment with a stated plan for rooflights, which are what people fall through. Collective edge protection before harnesses, a lock-off procedure for DC work, and operatives trained for height rather than merely insured for it.

Testing, commissioning and proving the array does what was modelled

Commissioning is where a careful installation separates itself from a fast one.

The DC side is tested string by string: continuity and polarity, open-circuit voltage against the calculated figure, operating current, insulation resistance. The AC side is inspected, tested and certified to BS 7671. Inverter grid protection settings are proved against the connection agreement, and anything that agreement limits is proved by test rather than assumed from a value on a screen.

Then measured output is checked against the modelled figure for the same irradiance, which is the only real test of whether the design was honest. One caveat. An array commissioned in December will not show design output, and the meaningful comparison arrives with the first full spring.

The handover pack to insist on before final payment

A commissioned system is not a finished job. The file is what lets another contractor work safely on the array years later, and what an insurer or a buyer's surveyor will ask to see.

• As-built roof layout and single-line diagram, with string numbering that matches the labels on the roof
• Electrical installation certificate to BS 7671, with the full schedule of test results
• Commissioning confirmation and the network operator's connection acceptance
• Structural appraisal, fixing specification, and the roof manufacturer's written position on the warranty
• MCS certificate where the system is MCS-certificated — the SEG export route needs one, and it is worth holding in any case; larger arrays exporting under a power purchase or negotiated supply agreement are evidenced differently
• Panel, inverter and mounting warranties, with the product and performance terms stated separately for the products actually supplied
• Insurance-backed workmanship warranty documentation (IWA), with the policy term stated on the certificate
• Monitoring portal logins registered to the building owner, with the installer added as a user
• Operating file, isolation procedure, and a labelled shutdown sequence for the fire service
• Health and safety file where CDM applies

If anything on that list that applies to your system is outstanding, hold the final payment until it arrives. The monitoring logins repay a close look, because registering them to the installer instead of the building owner is awkward to unpick later.

The things that most often delay an installation

Programmes slip for a short list of predictable reasons, and most are visible before contract if anyone looks.

• Grid connection. The network operator's answer sits outside everyone's control, which makes it the stage most likely to stretch a programme, because nothing done on site moves it along.
• Landlord consent. On a leased building this can outlast the connection application, because no statutory timescale applies and nobody at the managing agent is measured on it.
• Asbestos found at survey on a pre-2000 building, changing method, access and cost.
• A structural appraisal that requires strengthening, or restricts the array to part of the roof.
• A roof covering near the end of its life, which should be replaced before an array is fixed to it for decades.
• Listed building consent, a conservation area, or a street-facing array that falls outside permitted development.
• Weather. Roof work stops at wind speed limits, and a fortnight of February gales removes a fortnight from the programme.
• Internal capex approval on your own side, which is nobody's fault and still costs weeks.

One of these is worth testing before you spend anything. A tenant with a short remaining lease, and no prospect of the landlord contributing or extending, will rarely see a rooftop array repay inside their occupation. Better to settle that before the survey than after the design.