Solar panel temperature coefficient calculator: cell temperature and heat loss

A solar panel loses power as its cells heat up, at the rate of its temperature coefficient of maximum power (Pmax): -0.24 to -0.34 %/°C on four datasheets below. This calculator turns air temperature, irradiance, wind and mounting into cell temperature and heat loss. At 42.0 °C air, 1,000 W/m² and 1 m/s, a -0.29 %/°C module loses 14.3%.

What does the solar panel temperature coefficient calculator compute?

The calculator estimates cell temperature from the weather and the mounting, then applies the temperature coefficient. The calculator above takes five inputs:

• Air temperature, -10 to 45 °C.
• Plane-of-array (POA) irradiance, 0 to 1,200 W/m².
• Wind speed, 0 to 10 m/s.
• Mounting type, in four options.
• Temperature coefficient of Pmax (γ), -0.50 to -0.20 %/°C.

It returns the cell temperature (°C), the power change against a 25 °C cell temperature (%) and the DC power of a 1 MWp array (kW). The power change is negative when heat costs power, and below 25 °C the hint reads "Gained from cool conditions". The chart under the calculator shows the power change across air temperatures from -10 to 45 °C for your coefficient, with reference lines at -0.34 and -0.24 %/°C. In the text, a loss of 14.3% means a power change of -14.3%.

The model is the Sandia Array Performance Model (SAPM) module temperature model by King, Boyson and Kratochvil, published as Sandia report SAND2004-3535 in 2004 and implemented in pvlib-python:

T_module = E × exp(a + b × WS) + T_ambient
T_cell = T_module + (E / 1000) × ΔT
Power relative to 25 °C = 1 + (γ / 100) × (T_cell - 25)
Power change vs 25 °C (%) = (Power relative to 25 °C - 1) × 100
DC power (kW) = 1000 × (E / 1000) × Power relative to 25 °C

E is POA irradiance in W/m², WS is wind speed in m/s, and a, b and ΔT are set by the mounting. The mounting table below lists them, as in the Sandia report and pvlib.

What is the temperature coefficient of Pmax, and where is it on a datasheet?

The temperature coefficient of Pmax (γ) is the change in a module's maximum power, in percent, for each 1 °C change in cell temperature. It is measured against standard test conditions (STC) of 1,000 W/m² and 25 °C cell temperature, as the JinkoSolar datasheet states. It sits in the temperature block of the datasheet, beside the voltage and current coefficients and the nominal operating cell temperature (NOCT) or nominal module operating temperature (NMOT).

A module at -0.30 %/°C loses 0.30% of its power per °C above 25 °C, or 7.5% at a 50 °C cell. The value is a linear approximation, and First Solar quotes its figure for 25 °C to 75 °C.

What temperature coefficients do PERC, TOPCon, heterojunction and CdTe modules have?

Current datasheets list -0.24 %/°C for heterojunction (HJT) and -0.29 %/°C for TOPCon (tunnel oxide passivated contact). They list -0.32 %/°C for cadmium telluride (CdTe) thin film and -0.34 %/°C for PERC (passivated emitter and rear cell). Each row is one product, so read it as an example, not a technology average, and check the datasheet of the exact module.

Why is cell temperature higher than air temperature?

Cells turn only part of the sunlight into electricity. The rest heats the module until heat loss to the air balances it. In the SAPM, per the Sandia report, coefficient a sets the upper limit for module temperature at low wind and high irradiance. Coefficient b sets how fast it falls as wind rises.

NOCT is the cell temperature at 800 W/m², 20 °C air and 1 m/s wind. IEC 61215-2:2016 replaced it with NMOT, measured with the module delivering power in an open rack, which typically reads a few degrees lower. The 2021 edition removed the NMOT test from the IEC 61215 series, yet datasheets still list one or the other: 42 to 45 °C on the four above. The SAPM open rack glass/glass set gives 45.9 °C for those conditions.

Mounting matters because it controls airflow behind the module. The pvlib documentation describes three mounting types. Open rack is typical of ground-mounted fixed racking or single-axis trackers.

Close mount is typical of roofs with a restricted gap, and insulated back is typical of building-integrated systems or modules laid flat on the ground. The table shows each set at 42 °C air, 1,000 W/m², 1 m/s and -0.29 %/°C.

An insulated back runs 25.2 °C hotter than open rack glass/glass at these inputs. In open rack glass/glass, raising wind from 1 to 5 m/s lowers the cell from 74.3 to 68.1 °C and the loss from 14.3% to 12.5%.

What does Hungary's 42.0 °C heat record mean for a solar array?

At 42.0 °C air, a 1 MWp array with a -0.29 %/°C coefficient delivers about 857 kW at 1,000 W/m², not 1,000 kW. HungaroMet's records page lists 42.0 °C at Szécsény on 30 June 2026 as Hungary's highest air temperature. Its news item reports that it beat the 41.9 °C at Kiskunhalas on 20 July 2007.

The example applies the formulas to that air temperature with open rack glass/glass, 1,000 W/m² and 1 m/s. Irradiance and wind are assumed stress-case values, not measurements from that day. The model gives a back-surface temperature of 71.3 °C and a cell temperature of 74.3 °C, 32.3 °C above the air.

PERC and HJT differ by 4.9 percentage points, or about 49 kW per MWp. Only the coefficient changes between rows, since the cell temperature is the same.

The calculator opens on 42 °C (the HungaroMet record), 1,000 W/m² (the STC irradiance), -0.29 %/°C (the TOPCon datasheet) and open rack glass/glass (the pvlib ground-mount case). Wind starts at 1 m/s. The model's wind input is at 10 m height, whereas the 1 m/s in NMOT and NOCT is at module height. Here 1 m/s is used as a calm-air stress case, so the inputs together are not a typical hour.

Why does a hot July hour lose more than a cool, clear April hour?

Cell temperature follows air temperature almost one for one, while irradiance, and so power, is similar in both months. The table uses the hourly series from PVGIS 5.3 for Budapest (47.500 N, 19.040 E, 30° tilt facing south, 2005 to 2023). It combines PVGIS-SARAH3 irradiance with ERA5 reanalysis air temperature and 10 m wind. The SAPM open rack glass/glass set runs on every hour with POA irradiance of at least 800 W/m².

With near-equal irradiance, July's cells run about 11 °C hotter and lose about 3 percentage points more at -0.29 %/°C. Over a whole year the effect is smaller, because cool hours count as gains.

Weighted by irradiance across all sunlit hours of 2005 to 2023, the average loss is 2.8% at -0.24 %/°C and 3.4% at -0.29 %/°C. At -0.34 %/°C it is 3.9%. The hottest modelled cell in an hour with at least 800 W/m² is 67.1 °C (19 July 2007).

What this means for module choice and yield

Heat loss peaks in the hours that produce the most. At the stress inputs a 1 MWp array delivers 832 to 882 kW depending on the coefficient, while the yearly average loss in Budapest is 2.8% to 3.9%. A coefficient of -0.24 %/°C instead of -0.34 %/°C is worth 1.2 percentage points of annual DC energy in this model.

Mounting moves the result as much as technology does: close mount glass/glass loses 5.0 percentage points more than open rack glass/glass at the stress inputs. The coefficient is also one datasheet line among others, such as rated power, degradation and price.

Datasheet coefficients describe new modules, and the PV degradation calculator covers the decline over the years. Capacity factor vs performance ratio explains how to read a park's numbers. Data sources and conventions are on the methods page.

Limits of the calculator

• Steady state. The SAPM coefficients come from near thermal-equilibrium, clear-sky data, so clouds and the module's thermal mass are not modelled.
• Accuracy. The Sandia report puts module temperature accuracy at about ±5 °C, which it says changes module power by less than 3%.
• Wind. Wind direction is ignored, as in the model.
• One linear coefficient. First Solar quotes 25 °C to 75 °C, and the stress case sits just under that top end.
• Left out. Irradiance-dependent efficiency, spectrum, soiling, angle of incidence, mismatch, wiring and inverter losses, inverter clipping and degradation.
• Gridded data smooths extremes. The highest PVGIS air temperature for Budapest is 38.8 °C (20 July 2007), against the 40.7 °C HungaroMet measured at Budapest Pestszentlőrinc that day.
• Estimates only. Results explain the size of the effect. They are not engineering or financial advice.