WISC stands for Wind and Infrared Sensitive Collector — the ISO 9806 category for collectors without a glazing cover, which includes most PVT hybrid panels. Because no glass cover shields the absorber, performance depends measurably on wind speed and on infrared exchange with the sky, so WISC collectors are tested and listed on Solar Keymark certificates with wind-dependent coefficients rather than the standard glazed-collector curve.
What does WISC mean on a Solar Keymark certificate?
WISC is the ISO 9806 test category for Wind and Infrared Sensitive Collectors — collectors whose absorber faces the environment without a transparent cover. On a Solar Keymark certificate the designation tells you two things at once: the collector is unglazed (or the thermally equivalent construction), and its certified performance figures were measured with wind speed and sky temperature as explicit test variables. Most uncovered PVT hybrid collectors fall into this category, because the PV laminate itself forms the outer surface and there is no separate glass cover over an air gap.
Why are unglazed collectors "wind and infrared sensitive"?
Without a cover, the absorber exchanges heat directly with moving air and with the cold sky. Wind strips heat from the surface, so thermal output falls as wind speed rises; on clear nights the surface also radiates to a sky that is effectively far colder than ambient, which is why unglazed collectors can even dip below air temperature. ISO 9806 therefore characterises WISC collectors with coefficients that scale with wind speed, and Solar Keymark datasheets state performance at defined wind speeds (typically around 1.3 m/s for the reference condition) instead of a single still-air curve. This sensitivity is not a defect — it is exactly what makes an unglazed PVT panel useful as an air-coupled heat pump source, absorbing heat from wind and ambient air when the sun is down (see how a PVT panel feeds a heat pump).
Covered vs uncovered PVT: what does the certificate difference mean in practice?
| Aspect | Uncovered PVT (WISC) | Covered / glazed PVT |
|---|---|---|
| Outer surface | PV laminate exposed to the air | Glass cover with air gap over the laminate |
| Certified test category | WISC per ISO 9806 | Glazed liquid-heating collector |
| Typical delivery temperature | Low-grade (DHW preheat, pool, heat pump source) | Higher water temperatures for direct DHW |
| Wind influence | Explicit in certified coefficients | Minor |
| Night/ambient heat gain | Yes — works as an air heat exchanger | Negligible |
| Stagnation behaviour | Mild, PV-limited temperatures | Higher stagnation temperatures |
Which construction suits which system is covered in insulated, uninsulated or finned PVT.
How do I read the area figures on a Solar Keymark certificate?
Solar Keymark datasheets quote performance per gross area — the full outer envelope of the collector — and also state the aperture area where relevant. For WISC collectors the convention is to reference gross area, so when you compare an unglazed PVT panel against a glazed collector, make sure both figures use the same area basis; a per-aperture figure will flatter a glazed collector against a per-gross figure. The certificate also lists power output at defined irradiance (1000 W/m²) and temperature differences, which is more useful for system sizing than a single efficiency percentage.
Which certificates back a PVT panel — thermal, electrical or both?
Both sides need their own certification, because Solar Keymark covers only the thermal collector function. A properly certified PVT product therefore carries a Solar Keymark licence for the collector and IEC certification for the PV module. Solimpeks' PV-T hybrid panels, produced since 2008, hold Solar Keymark through Kiwa Cermet Italia under licence 16918 Rev.0, with the PV module certified under 16917 Rev.0 to IEC 61215 and IEC 61730-1. Subsidy schemes that fund PVT — Germany's BEG among them — require the Solar Keymark licence for the thermal side, which is why an uncertified hybrid can cost you the grant (see are PVT panels eligible for grants in 2026).
Frequently asked questions
Is an unglazed collector worse than a glazed one?
No — it is optimised for a different job. Glazed collectors hold higher water temperatures for direct hot water; unglazed WISC collectors excel at low-temperature loads such as pools, preheating and heat pump source circuits, where their air-coupling becomes an asset rather than a loss.
Where can I check a collector's Solar Keymark certificate?
The Solar Keymark database at solarkeymark.org is public: search by manufacturer or licence number and download the datasheet. Verify that the certificate number on the quote matches the database entry and that the certificate status is current.
Does WISC certification affect subsidy eligibility?
The category itself does not disqualify a collector — what matters is holding a valid Solar Keymark certificate, which several major schemes require for solar thermal and PVT funding. Some schemes additionally distinguish covered and uncovered collectors in their eligible-technology lists, so check the scheme's current rules.
Why does my PVT datasheet quote thermal power at a wind speed?
Because for a WISC collector wind speed materially changes output, ISO 9806 requires performance to be characterised with wind as a variable. Quoting power at a stated wind speed (commonly around 1.3 m/s) makes figures comparable between products — a number without a wind reference is not a certified WISC figure.