Short answer
As the cell temperature rises above 25 °C, a solar panel loses a certain share of its nameplate power for every degree; this share is written in the panel's datasheet as the temperature coefficient and is usually between 0.3 and 0.45 percent per degree for crystalline silicon panels. On a July afternoon in Şanlıurfa, the cell of a panel on a roof can reach around 70 °C; for a panel with a coefficient of 0.35 percent per degree, this means a loss of about 16 percent in instantaneous power. Even so, because the sun is much longer and stronger in summer, a panel produces much more energy in summer than in winter.
At what temperature is a panel's nameplate power measured?
The power on the label is measured at a cell temperature of 25 °C and irradiance of 1000 W/m². These conditions are called standard test conditions (STC). Cell temperature is the temperature of the silicon cells under the panel's glass, and in the midday sun it is much higher than the air temperature.
According to the 1929-2025 statistics of the General Directorate of Meteorology, the average maximum temperature in Şanlıurfa is 38.8 °C in July and 38.4 °C in August. On an afternoon when the air is 38 °C, it is impossible for the cell to stay at 25 °C. So expecting to see the nameplate power on the inverter screen at summer noon is not realistic.
How is the temperature loss calculated?
The loss is found by multiplying the temperature coefficient by the difference between the cell temperature and 25 °C. Use the value that appears in the panel's datasheet as "Pmax temperature coefficient" or "power temperature coefficient." In crystalline silicon panels this value is usually between 0.3 and 0.45 percent per degree; it varies with the cell technology.
To estimate the cell temperature, the NOCT or NMOT value in the datasheet is used. This value is the cell temperature the panel reaches under a certain irradiance, air temperature and wind; for crystalline panels it is usually given in the 40s of degrees. A common approximate formula is: cell temperature ≈ air temperature + (NOCT − 20) × irradiance ÷ 800.
The example below is for a panel with a coefficient of 0.35 percent per degree and a NOCT value of 45 °C. Take your own panel's values from the datasheet.
- Air temperature: 38.8 °C (July average maximum, MGM)
- Irradiance: 1000 W/m² (example for a clear midday hour)
- Cell temperature ≈ 38.8 + (45 − 20) × 1000 ÷ 800 ≈ 70 °C
- Temperature difference: 70 − 25 = 45 °C
- Power loss: 45 × 0.35 percent ≈ 16 percent
- In a 10 kW system, instantaneous power therefore drops to about 8.4 kW (not counting other losses)
Expert note: If you do the same calculation with a panel that has a coefficient of 0.30 percent, the loss comes to about 13.5 percent, and with a panel at 0.45 percent, about 20 percent. When comparing quotes, look at this coefficient as well as the panel power.
Despite the loss in summer, why does a panel still produce the most in summer?
Heat lowers instantaneous power, but the solar energy falling on the panel in summer is several times that of winter. The Ministry of Energy and Natural Resources' Solar Energy Potential Atlas (GEPA) gives the daily global radiation for Şanlıurfa as 6.58 kWh/m² in July and 1.80 kWh/m² in December. The difference is more than three and a half times; a 16 percent temperature loss does not close this gap.
The daily energy loss is also smaller than the loss at the midday moment. In the morning and evening hours, both the air and the panel are cooler, and the effect of the coefficient shrinks at those hours. The point of accounting for temperature loss is to estimate summer production realistically and not to mistake the midday value on the inverter screen for a fault.
How does the mounting method change cell temperature?
The more freely air can circulate behind the panel, the cooler the cell stays. The NOCT value is given for a mounting that is open at the back and gets wind. Panels that sit almost flat against the roof, with a few centimeters of gap underneath, heat up beyond this value; panels on a tall structure in a field generally run cooler.
On a flat roof, raising the panel off the concrete surface and leaving air corridors between rows helps summer efficiency. On a pitched roof, the gap between the roof tiles and the panel should not be made too narrow. The roof structure and wind load need a separate assessment; we covered structure and orientation in our rooftop panel article.
How are inverters and cables affected by heat?
The inverter can also reduce its power in the heat to protect itself. The datasheet of most inverters says that it reduces output power above a certain ambient temperature. An inverter hung on a wall that gets direct sun from the south or west may not be able to deliver all the energy from the panels to the grid on a summer afternoon.
So hang the inverter in a shaded place with airflow; do not put it in a closed cabinet that gets sun. Leave around it the clearance the manufacturer asks for. Using sun-resistant DC cables and not letting them get pinched in cable trays also reduces heating at connection points in the hot months.
Why is cold also accounted for in winter?
The flip side of the temperature coefficient appears in winter: a cold panel produces higher voltage. In MGM records, the lowest temperature measured in Şanlıurfa in February is minus 12.4 °C. On such a cold and sunny morning, the open-circuit voltage of the panel string rises above the label value.
For example, for a panel with an open-circuit voltage coefficient of 0.27 percent per degree, the 37.4-degree difference between 25 °C and minus 12.4 °C means a voltage increase of about 10 percent. When calculating how many panels will be connected in series in a string, this increase must not exceed the highest input voltage the inverter allows. In project design, this check is done with the panel and inverter datasheets.
What should a panel owner check in summer?
Compare the production in the inverter app with similar sunny days. If power at July noon is below the label value, that alone is not a sign of a fault. If there is a clear drop compared with previous days at the same temperature, there may be dust, shade or a connection problem in a string.
Make the comparison at the same hour and in similar weather conditions. The difference between a day after a dust storm and a clear day after rain comes from the dirt on the surface more than from temperature. If the app shows production by string, one of two strings facing the same direction constantly lagging behind is also a sign that needs maintenance.
In rooftop and field systems in Şanlıurfa, Desa Bilişim chooses panels and inverters according to the temperature values in the datasheets and plans the installation location according to summer conditions.
Frequently asked
Does it make sense to buy a panel with a low temperature coefficient?
In a hot place like Şanlıurfa, the difference in coefficient shows up in summer production. But the decision is not made on this value alone; the panel's power, efficiency and manufacturer support are also evaluated together. When comparing quotes, write down each panel's datasheet coefficient side by side.
Does cooling the panels with water in summer increase efficiency?
It cools the cell for a short time, but suddenly cooling hot glass with cold water can damage the panel, and hard water leaves marks on the surface. Cleaning is done in the cool hours, with soft water. Leaving an air gap at installation is a more lasting solution than regularly spraying water for cooling.
Why does the inverter not show the nameplate power at noon?
Nameplate power is measured at a cell temperature of 25 °C. At summer noon the cell is far above that, so power drops; cable, inverter and dirt losses add to it. If there is no sudden drop compared with previous days under the same conditions, this is normal.
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