Why Solar STC is not always STC...
11th April 2019

 In our industry it is universally accepted that STC (Standard Test Conditions) for a solar PV module are 1000W/2 of solar irradiance, 25C cell temperature and a light spectrum that closely simulates sunlight at AM 1.5G

It reality, it is often found that during the manufacturing process the STC requirements are not diligently met by internal quality control. This can be caused by deviations in the lamp spectrum, cell temperature, ambient temperature, irradiance levels etc. during the testing process.

This means the stated STC conditions are not always precise.

We therefore use a nominal value to more accurately represent the real world. This is what is called the nameplate capacity and we take this directly from the datasheet of the module manufacturer, it is our de facto value.

However in order to compare predicted performance with actual performance we transpose our measured values to STC....

Essentially we're trying to convert real world values to laboratory conditions...

To make matters more complicated the problems we have in the lab are multiplied in the field:

- The accuracy of our cell temperature reading is low as we are connected to the back sheet not directly to the cell

- We are not taking an air mass reading

- We are often testing a string, not a single module. This increases series resistance.

- In the field the temperature is often much higher or lower than STC.

All of the above increase the margin of error in our calculation from measured to STC.

To further compound issues, solar PV O&M engineers are under tight deadlines to take measurements and generate reports. This leads to rushing the testing process and not leaving enough time for more than one test per string.

Unfortunately there is no magic bullet to tell us if an array is performing as expected, that is why we should be intelligent and honest in the way we interpret our data.

We should consider several factors along side the transposed STC values:

- Is the insulation resistance low? According to the IEC 62446:2016 standard this should be at least 1МΩ

- Is the expected Voc and Isc in line with our measurements?

- How many modules are in the string, should we lower the string length to get more accurate readings.

- What is the shape of the I-V curve and what is the FF?

- Was my sample rate high enough for accurate results?

With all of this in mind do not rely solely on STC calculations and software. By considering all the variables we can give a more holistic approach to solar PV maintenance.

Michael Middlemast

Category Manager for Renewables

Seaward Solar


Seaward Solar offer free online PV O&M training with every solar PV tester purchase.

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