An uncovered (WISC) PVT panel delivers water at 10–40 °C, typically only 3–8 K above its inlet on a single pass, which is why it feeds a heat pump source circuit rather than a hot water cylinder directly. Insulated or glazed PVT reaches 50–70 °C instead, trading part of its annual electrical yield for that higher temperature.
What temperature does water actually leave a PVT panel at?
An uncovered PVT panel delivers 10–40 °C at the outlet in normal operation, and only 3–8 K more than whatever went in. Independent test campaigns record outlet temperatures between roughly 29 °C and 44 °C at irradiance of 400–800 W/m², and the fluid can run below ambient air temperature when the panel works as a heat pump source. That is the design intent, not a defect.
The number to design around is therefore not the outlet temperature but the temperature lift across the panel. At the nominal flow rate used for most PVT hybrid collectors — around 40–60 litres per hour per m² — the fluid picks up only a few kelvin per pass. A store reaches 45 °C because the pump circulates it through the array a hundred times, not because one trip through the absorber does the work.
Why does an uncovered PVT panel run cool on purpose?
Because electricity is worth more per kilowatt-hour than lukewarm water. Crystalline silicon loses 0.25–0.34% of rated power for every degree the cell sits above 25 °C: about −0.34%/K for mono PERC, −0.29%/K for TOPCon and −0.25%/K for HJT cells. Any measure that pushes water temperature up — glazing, insulation, low flow — pushes cell temperature up with it, and the electrical yield pays the bill.
Uncovered PVT collectors are classified as WISC (wind and/or infrared sensitive collectors) under ISO 9806. The term replaced "unglazed" in 2013 and is what appears on a Solar Keymark certificate. A WISC panel exchanges heat with wind, ambient air and the night sky as well as with the sun — which is precisely why it makes a strong heat pump source. It keeps harvesting after sunset, when a glazed collector has already gone cold.
When can a PVT panel reach 50–70 °C?
Only when it is insulated or glazed, and mainly in summer. Adding rear insulation and a covered air gap lifts the achievable outlet band into 50–70 °C, high enough to charge a cylinder directly. The price is a lower annual electrical yield and a far higher stagnation temperature. Match the collector to the job:
| Application | Outlet temperature to design for | Panel type |
|---|---|---|
| Heat pump source (brine circuit) | −10 to +25 °C | Uncovered WISC, often finned |
| DHW pre-heat into a lower coil | 25–45 °C | Uncovered or insulated |
| Direct DHW charging to 55–60 °C | 50–70 °C | Insulated or glazed PVT |
| Swimming pool heating | 22–30 °C | Uncovered WISC |
| Radiator circuit at 65–70 °C | Not achievable | Use thermal collectors instead |
Loads that need genuine high-temperature heat — process water, legacy radiators, industrial pre-heat — belong on flat plate collectors or evacuated tubes, not on PVT. Solimpeks builds its PV-T Hybrid Panels alongside the Wunder thermal range for exactly this reason: the temperature the load demands decides the collector, and no single panel wins at every temperature.
What is the hottest a PVT panel will ever get?
Field measurements put the stagnation temperature of a WISC PVT panel below about 75 °C, against 150–200 °C for a glazed flat plate collector. That is a genuine engineering advantage and it flows through the whole system: propylene glycol degrades rapidly above roughly 130 °C, so a PVT loop's heat transfer fluid ages far more slowly than a conventional solar loop's, and the expansion vessel can be sized without allowing for steam.
How do I read the outlet temperature off a datasheet?
Use the efficiency curve, not the marketing figure. A Solar Keymark tested WISC PVT panel typically shows a zero-loss efficiency around 0.55–0.60, a heat loss coefficient near 12 W/m²K and a separate wind coefficient — three numbers that ISO 9806 requires and that no brochure headline can override. Because the loss coefficient is high, useful thermal output collapses as soon as the fluid rises much above ambient. That curve is what tells you the panel will never charge a 60 °C cylinder on its own.
Feed the array into the source side of a heat pump instead, and the low working temperature becomes the asset: the panel stays cool, the PV yield stays high, and the heat pump sees an elevated source temperature all winter.
Frequently asked questions
Can a PVT panel heat my hot water to 60 °C on its own?
An uncovered WISC panel cannot; it works in the 10–40 °C band. Insulated or glazed PVT can reach 50–70 °C in summer, but a heat pump or immersion element still has to make the final lift to 60 °C for legionella control in most European climates.
What is the minimum operating temperature of a PVT panel?
With a propylene glycol mixture, PVT panels operate down to roughly −20 °C, and as a heat pump source they are routinely run below 0 °C on purpose. Running the fluid colder than ambient is what lets the panel pull heat from the air as well as from the sun.
Why is my PVT outlet only 5 degrees above the inlet?
That is normal and correct. At the nominal flow of 40–60 L/h per m², a single pass adds only a few kelvin. Store temperature builds through repeated circulation, so judge the system by the tank temperature after a day, not by the panel's instantaneous delta-T.
Does a hotter PVT panel produce less electricity?
Yes. Cell output falls by 0.25–0.34% per degree above 25 °C, so a panel running at 60 °C gives roughly 9–12% less power than the same panel at 25 °C. Keeping the fluid cool is what produces the electrical bonus PVT is sold on.