Calculate temperature losses of solar panels

Module temperature follows the Faiman model, T_m = T_a + G / (u₀ + u₁ · W), and power follows the Huld efficiency model at the actual operating point — not the datasheet straight line. The loss is referenced to the power at STC cell temperature rather than the nameplate rating, so it contains only the temperature effect and not losses that simply come from lower irradiance.

It is the most important single figure missing from the datasheet: at 25 °C air, 1000 W/m² and 1 m/s wind, the Faiman model gives 55.2 °C free-standing but 68.1 °C roof-mounted or integrated — a 12.9 K difference with identical modules. For panels glued flat onto a van or boat roof this is the most relevant number in the whole yield domain; the calculator prices the penalty in kWh and cents per year.

The Huld fit is a regression and holds roughly from 200 W/m² upwards; below that the calculator warns. Below about 8 W/m² the fit even extrapolates into negative efficiency — power is clamped to zero there and the range is flagged. Thermal inertia during fast irradiance changes, forced ventilation, partial shading, bifaciality and degradation are not covered.

At the reference point (25 °C air, 1000 W/m², 1 m/s wind) the Faiman model gives: free-standing 55.2 °C, roof/glued 68.1 °C — 12.9 kelvin from mounting alone, with identical modules. With c-Si coefficients that is about 6.5% power difference. For camper and boat roofs with glued-on modules this is the single most important number in the whole yield section — the calculator outputs it in kWh and euros per year so you can weigh it against the cost of rear ventilation.

The datasheet value γ (e.g. −0.38 %/°C) is a straight line that only knows temperature. The Huld model additionally captures the irradiance effect: at low irradiance, efficiency drops even without heat. At 1000 W/m² both lie close together; at 200 W/m² they diverge clearly — and the calculator warns when the deviation exceeds 5%. The curve chart shows both models overlaid.

Because at 600 W/m² the power is below the rated value anyway — that has nothing to do with temperature. Referencing the loss to rated power attributes irradiance losses to the temperature coefficient. This calculator keeps them separate: the reference is the power at the same irradiance but 25 °C cell temperature. The stated percentage is therefore purely the temperature share.

Yes — the temperature coefficient works in both directions. On a clear frosty day (−10 °C air, 1000 W/m², 1 m/s wind, free-standing) the model gives a module temperature of 20.2 °C: a 440 Wp panel then delivers 450 W, a good 2% above its nameplate rating. On a 30 °C summer day the same irradiance yields only 370 W. Winter still produces less energy overall, because it lacks irradiance — cold improves efficiency, not sunshine. The temperature coefficient is an efficiency argument, not an annual-yield argument.

On a hot summer day (30 °C air, 1000 W/m², 1 m/s wind) the model gives: free-standing 60.2 °C module temperature and 15.9% temperature loss, roof-mounted or glued-on 73.1 °C and 21.5%. At 35 °C air the figures rise to 18.1% and 23.6%. These are operating-point values at the extreme — over a full year the temperature loss is much smaller, because such hours are rare. That is exactly why this calculator evaluates the stated operating point instead of a datasheet flat rate.

The most effective “cooling” is not a gadget but rear ventilation: free-standing instead of glued-on lowers module temperature by 12.9 K at the reference point and gains about 25 W on a 440 Wp panel — projected to roughly 25 kWh or €7 per panel per year at 1000 full-load hours. Wind also cools measurably: 5 m/s instead of 1 m/s brings a free-standing module from 55.2 down to 42.3 °C (+25 W). Active water or forced cooling is not modelled here; measured against a few euros per panel per year it rarely pays off.

NOCT (Nominal Operating Cell Temperature, typically 42–46 °C for crystalline silicon) is the module temperature at 800 W/m², 20 °C air, 1 m/s wind with an open back. At exactly that point the Faiman model gives 44.2 °C free-standing — consistent with datasheet values. But NOCT knows neither wind speed nor mounting type: a glued-on module already sits at 54.5 °C at the same point. For roof and glued mounting NOCT is therefore systematically optimistic; Faiman uses its own coefficients (u₀, u₁) per mounting type.