Why the residential comparison sites do not answer a farm question
Search for a solar panel comparison and the first page is written for houses: integrated panels, plug-in kits, off-grid lists. A 100 kW array on a portal-frame shed has nothing in common with that brief. The modules sit over livestock that breathe out ammonia, over grain dust, on fibre cement or profiled steel fixed to purlins, in open country with no shelter from wind or hail, on a three-phase supply that the distribution network operator must approve. The brand name tells you very little about how a module copes with any of that. The test results, the certificates and the small print do.
One point before the tests. Kiwa PVEL, the independent test laboratory behind the annual module reliability scorecard, states that modules with the exact same model type can be manufactured from completely different bills of materials, and that certification tests are typically not rigorous enough to catch the difference. A comparison therefore has to be of the specific bill of materials on your quote, not of the brand.
Test 1: independent reliability results
The 2026 Kiwa PVEL PV Module Reliability Scorecard, its twelfth edition, extends the certification tests far beyond the minimum: thermal cycling to 600 cycles instead of 200, damp heat for 2,000 hours instead of 1,000, mechanical and hail stress sequences, potential-induced degradation, light-induced degradation and, new for 2026, ultraviolet-induced degradation. Its headline findings are the opposite of reassuring: 87% of manufacturers and 61% of bills of materials had at least one test failure, up from 83% and 59% the year before, and for the first time no model achieved Top Performer status across every test.
Forty-three manufacturers are listed as Top Performers in at least one test, and fifteen have a model that is a Top Performer in seven of the eight. Names on the 2026 list that are familiar in UK farm quotes include Jinko, Trina Solar, JA Solar, Qcells, LONGi, Canadian Solar, Astronergy, Aiko Solar, DMEGC Solar and Phono Solar; the full list and the model numbers behind it are searchable at scorecard.pvel.com. Being on the list is the start of the check, not the end of it: ask the installer which test categories the exact model passed and for the product qualification report if the project is large enough to justify it.
The scorecard also explains two failure modes that matter on an agricultural roof. Thirty-seven per cent of bills of materials had at least one failure in thermal cycling, a record, and more than 70% of the modules that lost over 2% of their power after the test used a thin front encapsulant. Delamination failures tripled year on year. Both are the kind of defect that shows up after several winters, long after the installer has been paid.
Test 2: ammonia and salt-mist certificates
Two environmental tests exist precisely for farm and coastal sites, and neither is part of the standard certification. IEC 62716 is the ammonia corrosion test. TUV Rheinland describes it as the test for systems used in agricultural environments, for example on the roofs of livestock buildings, where ammonia from sprays and liquid manure attacks the glass, frames and connections. The module sits in an ammonia atmosphere for 20 cycles of 24 hours, 480 hours in all, and passes only if it shows no obvious visual defects and loses no more than 5% of its maximum output. For a poultry house, a pig unit, a cubicle shed or a parlour, ask for the certificate for the specific model and refuse a module without one.
IEC 61701 is the salt-mist test for coastal sites. It runs for eight cycles of seven days, 56 days in total, with the same pass criteria of no visible defects and no more than a 5% drop in output. Farms within reach of sea spray, which in practice means much of the Welsh, Cornish, Scottish and East Anglian coast, should treat it the same way. The research behind this is set out in do livestock buildings need ammonia-resistant solar panels.
Test 3: mechanical margins for an exposed roof
Farm roofs are exposed. RC62, the fire-safety code of practice for commercial PV published by the Fire Protection Association, notes that systems are designed to accommodate wind speeds of 54 metres per second (120 mph) but that higher speeds have been recorded at exposed, higher-altitude UK locations, and that gaps between panels and roof trap drifting snow and ice. The module build matters for that. In Kiwa PVEL's mechanical stress sequence every module that completed the test lost under 2% of its power, but 30% of bills of materials had a failure during it and 26% had at least one broken module; for modules of 2.5 to 3.0 m², the breakage rate was 6.9% with frames of 35 mm or deeper and 17.9% with shallower frames.
Hail tells the same story. Standard modules with 2.0 mm glass on both faces broke 43% of the time at 40 mm hail and 61% at 45 mm, while only 7% of thicker-glass modules, those with 3.2 mm front glass over a backsheet, 2.5 mm glass on both faces, or 3.2 mm over 2.0 mm, broke at 45 mm. A comparison that lists efficiency and price but not frame depth and glass thickness has left out the two numbers a farm buyer most needs.
The fixing kit is tested separately. MCS 012 is the product standard for pitched-roof installation kits; it covers resistance to wind uplift, reaction to fire and resistance to water penetration, and a certified kit carries a certificate from a body such as the BBA that states the uplift resistance. On fibre cement and profiled steel, ask which MCS 012 certified kit is specified and how the fixings reach the purlins. Where the sheets are asbestos cement the roof decision comes first, which is covered on solar panels on asbestos cement farm roofs.
Test 4: the warranty wording, not the headline number
Every datasheet carries two warranties: a product warranty against defects and a performance warranty promising a minimum output along a degradation curve. The headline years are the least useful part. Kiwa PVEL's methodology note puts it plainly: module warranties also often have shortcomings that leave site owners exposed to significant financial losses. What to read instead: which legal entity honours the warranty in the United Kingdom, how output is measured for a performance claim and who pays for the measurement, whether the warranty requires a certified installer or registration within a time limit, and whether a replacement includes labour, shipping and the loss of generation while the module is out. The layers, and how they interact with the installer's own workmanship cover, are explained on solar panel warranty.
Test 5: the performance characteristics that matter in Britain
Two numbers from the scorecard translate directly into kilowatt-hours on a UK roof. The temperature coefficient of power: the median for TOPCon modules improved to −0.29% per degree Celsius, against −0.32% for PERC and −0.27% and −0.26% for back-contact and heterojunction cells. The low-light response: the median relative efficiency at 200 W/m² compared with 1,000 W/m² was −3.5% for PERC, −3.6% for HJT, −3.8% for TOPCon and −5.4% for back-contact modules, which matters on an overcast March afternoon in Cumbria rather than a July noon. Bifaciality only pays where light reaches the rear face; on a close-fitted pitched roof it does not, on a canopy or open-sided building it can. The yield side of the comparison, region by region, is in PVGIS yields for farm roofs by region.
What a comparison cannot tell you: the installed cost
The module is a fraction of the price. The Department for Energy Security and Net Zero publishes the installed cost of every MCS-registered system; the May 2026 release covering 2025/26 gives a mean of £1,266.84 per kW (median £1,203.85) for installations of 10 to 50 kW on a total-installed-capacity basis, and £1,167.29 per kW for non-domestic installations in that band. Arrays above 50 kW are outside the MCS database and therefore outside the statistics. Those figures, and what moves a farm quote above or below them, are on solar panels for farm buildings cost.
The figures on this page and where they come from
| Claim | Figure | Source |
|---|---|---|
| Manufacturers with at least one test failure | 87% of manufacturers, 61% of bills of materials | Kiwa PVEL, 2026 PV Module Reliability Scorecard executive summary |
| Top Performer manufacturers | 43 in at least one test; 15 in seven of eight; 5 models passed all reliability tests | Kiwa PVEL 2026 Scorecard |
| Ammonia test | 20 cycles × 24 h = 480 h; pass = no visual defects and ≤5% output loss | TUV Rheinland, corrosion testing of PV modules (IEC 62716:2013) |
| Salt-mist test | 8 cycles × 7 days = 56 days; same pass criteria | TUV Rheinland (IEC 61701:2020) |
| Frame depth and breakage (2.5–3.0 m² modules) | 6.9% with frames ≥35 mm; 17.9% with frames <35 mm | Kiwa PVEL 2026, mechanical stress sequence |
| Hail breakage | Standard 2.0/2.0 mm glass: 43% at 40 mm, 61% at 45 mm; thicker-glass modules 7% at 45 mm | Kiwa PVEL 2026, hail stress sequence |
| Design wind speed | 54 m/s (120 mph), higher recorded at exposed UK sites | RC62 (FPA/RISCAuthority, MCS, Solar Energy UK), 2023, clause 5.4.10 |
| Temperature coefficient (median) | TOPCon −0.29%/°C; PERC −0.32; xBC −0.27; HJT −0.26 | Kiwa PVEL 2026, PAN performance |
| Bifaciality (median) | PERC 70.2%; xBC 70.8%; TOPCon 76.5%; HJT 86.2% | Kiwa PVEL 2026, PAN performance |
| Installed cost, 10–50 kW, 2025/26 | Mean £1,266.84/kW, median £1,203.85/kW; non-domestic £1,167.29/kW | DESNZ, Solar photovoltaic (PV) cost data, published 28 May 2026 (MCS installation database, TIC basis) |
| Fixing-kit certification | MCS 012: wind uplift, reaction to fire, water penetration | MCS 012 product certification scheme requirements |
How to use this on a quote
- Ask for the exact module model number and manufacturer part code on the quote, and check it against the Top Performer search at scorecard.pvel.com.
- For any building housing livestock, ask for the IEC 62716 certificate for that model; within reach of sea spray, the IEC 61701 certificate too.
- Ask for frame depth and glass build. On an exposed roof, prefer frames of 35 mm or more and the thicker-glass constructions.
- Ask which MCS 012 certified fixing kit is specified and how it fixes to the purlins; on asbestos cement, settle the roof first.
- Read the warranty documents, not the summary: the UK entity, the measurement method, the registration conditions and what a replacement includes.
The questions above sit alongside the thirty in the farm solar installer checklist; for how to check the installer rather than the panel, see agricultural solar panel installers, and for the inverter side of the same decision, solar inverters for farm buildings.
Test results compare modules on day one; for how output changes over twenty-five years, see how long solar panels last on a farm roof.
Solar panel comparison questions from farms
Which solar panel brand is best for a farm roof?
There is no single best brand. The 2026 Kiwa PVEL Reliability Scorecard names 43 manufacturers as Top Performers in at least one test and only five models passed every reliability test, and the same model number can be built from different bills of materials. Compare the specific module on the quote: its test results, its ammonia or salt-mist certificates where the building needs them, its frame depth and glass build, and the warranty wording.
Do livestock sheds need ammonia-resistant panels?
Ask for modules certified to IEC 62716. The test exposes the module to ammonia for 20 cycles of 24 hours and passes it only with no visible defects and no more than a 5% loss of maximum output. TUV Rheinland describes it as the test for modules on the roofs of livestock buildings, where ammonia from slurry and sprays corrodes frames and connections.
Does thicker glass matter on a farm building?
In Kiwa PVEL hail testing, standard 2.0 mm glass-glass modules broke 43% of the time at 40 mm hail and 61% at 45 mm, while only 7% of thicker-glass modules broke at 45 mm. Exposed agricultural roofs with no shelter from hail or wind benefit from the thicker build and from frames of 35 mm or more, which broke far less often in mechanical load tests.
What does a 25-year performance warranty cover?
A performance warranty promises a minimum output along a stated degradation curve. Kiwa PVEL warns that module warranties often have shortcomings that leave site owners exposed. Read who honours the warranty in the UK, how output is measured for a claim, whether the installer must be certified, and whether labour and shipping are included. The product warranty, which covers defects, is a separate and usually shorter term.
Are bifacial panels worth it on a farm roof?
Only where light can reach the back of the module. On a close-fitted pitched roof there is little gain. On a canopy, an open-sided structure or a ground frame over a light surface the rear side adds output, and Kiwa PVEL measured median bifaciality of 76.5% for TOPCon and 86.2% for HJT modules. Ask the installer to model it with and without the rear-side gain.