Standard Test Conditions (STC)
The calibration of solar modules involves determining electrical parameters such as the maximum possible power, the short-circuit current and the open-circuit voltage. As these values depend e.g. on the temperature and irradiance, the measurement conditions must be precisely defined. Everything that can influence the measurement results must be specified precisely so that two different laboratories obtain the same results. According to IEC TS 61836:2016 (Paragraph 3.4.16.5) and IEC 60904-3:2019, the following three measurement conditions traditionally apply to the standard test conditions:
1. Spectrum at air mass AM1.5, defined from 280 nm to 4000 nm.
2. Irradiance 1000 W/m²
3. Module temperature 25°C
Since the 4th edition from 2019 a further influencing parameter has been precisely defined in the IEC 60904-3 standard:
4. The entire radiation should hit the solar device perpendicular to the surface.
In practice, only 90% of the radiation hits perpendicular on a sun orientated solar device, the so-called direct radiation. The diffuse radiation, which hits the solar device from all directions, accounts for around 10%. This includes the circumsolar radiation, the sky blue and also the radiation reflected from the ground (albedo). As it would be very difficult and costly to reproduce this directional distribution in the laboratory, it was decided to irradiate 100% of the radiation vertically onto the solar device. This not only makes it easier to compare measurements from different laboratories, but also makes the solar devices more cost-effective due to the simpler measurement conditions, as each solar cell and each solar module is measured once at the end of the manufacturing process and is then classified according to the results. If this measurement were very time-consuming, the modules would be more expensive.
Nevertheless, it is possible to specify more realistic values for the solar modules. For this purpose, the energy rating was developed, in which the yields of a module are specified for standardised, climate zone-dependent annual profiles. However, it is not necessary to measure each module individually, but only one module per type. The individual module can then be scaled accordingly with the result of the individual test carried out anyway according to standard test conditions in order to obtain its energy rating values. The energy rating takes even more influencing variables into account, e.g. even the wind speed, which influences the module temperature and thus indirectly the efficiency of a solar module. In addition, the Energy Rating does not just specify a single measurement condition, but an entire year with hourly resolution, i.e. 365x24 = 8760 measurement conditions. And not just once, but 6 times, for 6 different climate zones.
Why is a temperature of 25° C specified in the standard test conditions, even though a module reaches temperatures of over 50° C in the sun?
This is actually an unrealistic combination: in a solar park, a module with an irradiation of 1000 W/m² will practically never have a temperature of only 25° C. However, this combination was chosen in order to consume less energy in production and thus be able to produce the modules at lower cost and with a lower energy footprint. As described above, all solar cells and solar modules must be measured at the end of their production process. This is done with a flasher that irradiates the module for a fraction of a second. During this short time, the module practically does not heat up. However, if you wanted to measure it at 50° C, you would have to preheat it accordingly, which would cost time and energy. Therefore, in the 1980s, 25° C was decided upon as the standard test temperature, as this is a typical temperature in the production halls and therefore the module does not have to be heated up in a lengthy and energy-intensive process.