How to Judge the Commercial PV Panels in Your Quote

Start with the roof, not the panel

Nobody buying a roof full of panels wants to become an expert in them, and you do not have to be. Four figures on the datasheet do most of the work. Most quotes name a brand and a wattage in bold, as though that settles the argument. It settles nothing. What matters is whether the panel's format, load rating and warranty suit your roof, and whether the quote gives you enough to check.

One number frames everything that follows. A kilowatt-peak of modern panels needs roughly 5 to 6 m² of unshaded roof once spacing, walkways and edge setbacks are allowed for, and produces something like 900 to 1,050 kWh a year in the UK. A 2,000 m² industrial roof with clear runs therefore supports around 330 to 400 kWp. Hold that figure: it tells you whether a quoted system size is plausible before you open a specification sheet.

Wattage, format, and how much capacity actually fits

Rated power, printed as Pmax, is measured at 1,000 W/m² irradiance and a 25°C cell temperature. Current commercial modules commonly run between 400 and 650 W in a frame of roughly 1.9 to 3.1 m². Divide one by the other and you have the figure that decides a constrained roof: watts per square metre.

That is all module efficiency is. Commercial photovoltaic panels advertised at 22% deliver 220 W/m² of module area; a 20% panel delivers 200 W/m². Current modules commonly sit between 20 and 23%, which is 200 to 230 W/m². Percentages invite a brand argument, while watts per square metre invite arithmetic. Across 800 m² of module area those two differ by about 16 kWp, which is worth paying for on an office roof that runs out before the demand does.

Format has a second, quieter effect. Larger modules cut the number of clamps, connectors and lifts per 100 kWp, which shows up in the labour line. Their rail spans and clamping zones also differ from smaller formats, so changing module size late in the programme is not a like-for-like substitution.

The two warranties, and how to compare them honestly

Two separate promises hide behind the word warranty. The product warranty covers the panel as an object: manufacturing defect, delamination, a failed junction box, frame corrosion. The performance warranty covers output, and it is the one with money attached to it.

A performance warranty has a shape rather than a single number. There is normally a first-year allowance of around 1 to 2%, then a linear annual decline of about 0.4 to 0.55%, landing on a guaranteed percentage of nameplate at year 25. Read the endpoint, then check it against the slope: 1% in year one plus 0.5% a year for 24 years leaves 87%. A guarantee of 87 to 89% at year 25 is genuinely strong, while the low eighties is ordinary. On a 250 kWp array, five percentage points is close to 12,000 kWh a year.

Then apply the sceptical test. A warranty is a claim against a company that still has to exist in 2051. Ask who administers claims in the UK, whether the cover is backed by third-party insurance, and what a claim involves. Neither warranty pays the labour or access equipment needed to swap a module on a roof. That is what an insurance-backed workmanship warranty is for, with the policy term stated on the certificate.

Mechanical load rating: the line that decides an industrial roof

This is the specification most buyers skip and the one most likely to rule a panel out. Datasheets quote tested loads in pascals, one figure for downward load and one for uplift. Those are test figures, not working limits. The design load is the tested load divided by a safety factor of 1.5, so a supplier quoting the raw test figure as the design load is either careless or hoping you are.

Two loads have to be checked against it. Wind uplift, calculated to BS EN 1991-1-4, varies with building height, site altitude, distance from the coast, terrain and, above all, where on the roof each module sits. Corner and edge zones carry local suction several times higher than the middle of the roof, so a competent layout keeps modules out of them or fixes them differently. Snow load, to BS EN 1991-1-3, governs at altitude and across much of northern Britain.

How the panel is held matters just as much. Certified loads apply only at the clamping positions used in the test. Move a clamp along the frame, or switch from short-edge to long-edge mounting, and the permitted load can drop sharply, taking the warranty with it. On metal industrial roofs, non-penetrative clip-fix preserves the existing roof warranty. It transfers uplift into the standing seam, though, so seam pull-out capacity can govern instead of the panel. Above roughly 1,000 m² of array, expect a structural survey, and on a pre-2000 building, an asbestos management survey first.

Fire class, insurers and the rest of the roof

A module carries its own fire classification, but the question an insurer asks is about the whole system. A well-rated module over a combustible insulation core does not give you a well-rated roof, and on older industrial units that build-up is often the real exposure. Ask for the module's classification, then ask for evidence the array has been assessed against your actual roof construction.

Four other items belong on the same list: how DC cabling is routed and contained, whether the design includes arc-fault detection or module-level shutdown, how the fire service isolates the array, and whether panels compromise smoke vents or sprinkler coverage. Module-level shutdown is not a UK regulatory requirement. Some insurers ask for it anyway, and it costs far less designed in than retrofitted.

Tolerance, flash tests and what arrives on site

Every module is flash-tested at the end of the production line. The test records measured Pmax, open-circuit voltage, short-circuit current and maximum power point against the serial number, and the manufacturer can supply that data for the pallets delivered to your site.

Ask for it. If you have bought 250 kWp, the flash report is how you prove that 250 kWp is what arrived. Positive-only tolerance, written as 0/+5 W, means every module meets or beats its nameplate rating. A symmetrical tolerance means some of what you paid for was never in the box. The data has a second use: grouping modules of similar measured current into the same string trims mismatch loss.

Nameplate power is measured under conditions a British roof almost never delivers together. The NMOT figure on the same datasheet, taken at 800 W/m² and a realistic cell temperature, typically lands around 24 to 26% below nameplate, and it is the closer analogue to a warm July afternoon. That is not a defect. It is why a properly modelled yield estimate never equals nameplate multiplied by hours of daylight.

Where panel choice barely matters

On a large warehouse roof with more area than the building's demand can absorb, and an export limit set by the network operator, the panel is close to irrelevant to the business case. That is worth saying plainly, because it is the opposite of what a panel-led sales pitch tells an estates manager.

Four other things move the numbers far more. How much generation you consume on site, which lands at 55 to 75% for a daytime-occupied building without storage and 80 to 95% with it. How the array is laid out around shading, plant and roof lights. The condition of the roof covering, since eight years left on a covering makes a 25-year panel warranty academic. And the grid connection, because the network operator's answer can cap the sensible system size, with the threshold sitting at roughly 50 kW, or 3.68 kW per phase.

Put bluntly: a 15% difference in panel quality moves payback by months, while a 15% error in the consumption assumption moves it by years. Some buildings should not have solar: a short lease with no landlord consent, a heavily overshadowed roof, a covering due for replacement, or a load profile concentrated after dark will each undo an otherwise sound case. You should hear that before you pay for a survey, not after.

Questions that expose a weak quote

Put these eight in one email to every supplier on your shortlist. The answers, and the silences, will sort the list faster than the prices do.

• Which exact module model and wattage, and can I have the datasheet and IEC certificates?
• What is the guaranteed output at year 25, and what first-year allowance sits behind it?
• What tested mechanical load, at which clamping positions, and does the layout stay inside them?
• Has wind uplift been calculated for this site, including the corner and edge zones?
• What is the module's fire classification, and has the array been assessed against this roof's build-up?
• Will I receive flash-test data for the modules actually delivered?
• If a module fails in year 12, who pays for the labour and access equipment?
• What changes in this quote if the network operator limits export?

A commercial PV panel provider who answers all eight in writing has done the design work. One who answers three and offers a discount instead has not, and that discount is usually the cheapest thing they will ever give you. When you do compare prices, reduce every quote to £ per kWp, which our [commercial solar cost guide](/cost/) works through.