NOCT (TONC): what it is and how it affects photovoltaic performance
NOCT, which stands for Nominal Operating Cell Temperature, is a parameter used in the photovoltaic sector to characterise the thermal behaviour of a solar module under reference operating conditions. In Spanish, it is commonly known as TONC, which stands for ‘Nominal Operating Cell Temperature’.
The NOCT allows us to estimate the temperature that the photovoltaic cell may reach when the module is operating under conditions that are closer to real-world operation than those used in STC tests. This data is particularly important because the operating temperature directly influences the voltage, power and energy yield of the photovoltaic string.
Generally speaking, a lower NOCT indicates that the module has a greater capacity to dissipate heat under reference conditions. However, the actual operating temperature of a plant will also depend on factors such as irradiance, ambient temperature, wind speed, module technology and mounting configuration.
What does NOCT or TONC mean in photovoltaics?
The NOCT concept describes a characteristic temperature of the photovoltaic module obtained under specific environmental and installation conditions. It should not be interpreted as the maximum temperature the panel will reach, nor as a constant temperature during operation. Traditionally, NOCT has been associated with reference conditions similar to the following:
- Solar irradiance: 800 W/m².
- Ambient temperature: 20 °C.
- Wind speed: approximately 1 m/s.
- Installation: the module is installed in an open configuration that allows air to circulate.
These conditions enable the thermal behaviour of different modules to be compared using a common reference. The IEC 60904-5 standard sets out methods for determining the equivalent cell temperature and its application in determining the NOCT. For further information on the relevant standards, please refer to the IEC 60904-5 on equivalent cell temperature and NOCT.
It is important to understand that the NOCT does not necessarily represent the temperature that the module will reach in a specific photovoltaic plant. At a site with high irradiance, high ambient temperatures and poor ventilation, the temperature of the cells may far exceed the value specified as NOCT.
Difference between NOCT and STC conditions
NOCT should not be confused with STC (Standard Test Conditions), which are used to determine the nominal power output of a photovoltaic module. STC conditions establish a standardised test environment that allows the electrical characteristics of different modules to be compared. Key parameters include an irradiance of 1,000 W/m² and a cell temperature of 25 °C.
The fundamental difference is that, under STC conditions, the cell temperature is fixed at 25 °C, whilst NOCT aims to characterise the temperature reached by the module under specific environmental conditions. Consequently, although a module may have a specific rated power under STC conditions, during actual operation its cells may frequently operate at temperatures considerably higher than 25 °C.
This difference is significant because, in silicon photovoltaic modules, an increase in cell temperature normally leads to a reduction in voltage and in the maximum available power. The temperature coefficients specified in the technical data sheet enable this behaviour to be quantified.
How does NOCT affect the performance of a photovoltaic module?
The NOCT is useful for understanding how a module may behave thermally once installed. The temperature of photovoltaic cells depends mainly on the solar radiation received, the ambient temperature, the wind, and the ability of the module and its mounting structure to dissipate heat. As a simple approximation, the cell temperature can be estimated from the ambient temperature, the irradiance and the NOCT value. A commonly used simplified expression is:
Tc ≈ Ta + [(NOCT − 20) / 800] × G
Meaning:
- Tc: estimated temperature of the photovoltaic cell in °C.
- Ta: ambient temperature in °C.
- NOCT: nominal operating temperature of the cell in °C.
- G: incident irradiance on the module in W/m².
This expression should be understood as an approximation. The thermal models used in professional simulations may incorporate other variables such as wind speed, module efficiency, radiation absorption and mounting configuration. The PV Performance Modeling Collaborative de Sandia National Laboratories It features a specific NOCT-based model that takes into account, precisely, the irradiance on the module’s surface, ambient temperature, wind speed, efficiency and mounting configuration.
Ventilation is particularly important. A module installed on an open structure can dissipate heat from both sides, whilst a module situated very close to a roof may have a higher operating temperature due to reduced air circulation. For this reason, NOCT is a useful thermal benchmark, but it is no substitute for a detailed temperature model when estimating the energy output of a photovoltaic plant.
NOCT, TONC and NMOT: which term is currently used?
Although NOCT continues to appear in technical data sheets, engineering documentation, simulation tools and photovoltaic literature, regulatory terminology has evolved alongside the development of new characterisation methods. Nowadays, it is also common to come across the term NMOT, which stands for Nominal Module Operating Temperature. The shift from ‘Cell’ to ‘Module’ reflects a thermal characterisation focused on the behaviour of the complete module under reference conditions.
The international standard IEC 61853-2:2026 It sets out procedures for determining the Nominal Module Operating Temperature and assessing the behaviour of photovoltaic modules under specific environmental and installation conditions. Consequently, it is currently possible to encounter both NOCT and NMOT, depending on the manufacturer, the technical data sheet, the standard used or the simulation tool. In Spanish, TONC continues to be used as the translation for Nominal Operating Cell Temperature.
From a photovoltaic engineering perspective, these parameters are useful for interpreting the module’s thermal behaviour, estimating its operating temperature and assessing the power losses associated with cell temperatures higher than standard measurement conditions.