Understanding STC In Solar Panels: PV Test Conditions Explained

In solar panel specification sheets, you will see specs measured at STC. These are the Standard Test Conditions we measure all solar panels in the lab. In some cases, you also have NOCT or NMOT specs listed. Here we will explain exactly what STC means for solar panels.

Alright, let’s start at the start:

Whenever a new tech like photovoltaic cells (PV cells) comes along, a number of manufacturers will start producing solar panels from them.

In order to check which solar panels are the best (because every manufacturer will say they have the best solar panels) and have a well-regulated and transparent market, we need a way to put them on the same common denominator.

In the case of PV cells and solar panels, we needed to devise a set of test conditions all solar panels should be tested at. That’s why the world’s regulatory authority on electrical and electronic devices – the International Electrotechnical Commission or IEC – proposed the first set of test conditions in a 1993 outline.

These test conditions are commonly referred to as STC or Standard Test Conditions for solar panels. The main goal of Part 1: Test requirements in the latest 2021 overhauling IEC 61215-1:2021 document titled “Terrestrial photovoltaic (PV) modules – Design qualification and type approval” is to answer the following 3 specific questions:

  1. “At what solar irradiance should solar panels be tested?” As we know, the amount of electricity solar panels produce heavily depends on how much sun we get.
  2. “What should the PV cell temperature be during a solar panel test?” The efficiency of solar panels depends on cell temperature. For example, a very hot 120°F solar panel will usually produce less electricity than at a milder 80°F temperature. Here is a quick solar panel temperature vs. efficiency chart that illustrates this relationship well.
    standard test conditions or stc include measuring pv module temperature
    Solar technicians will measure the solar panel temperature before measuring power output, voltages, and relevant currents.
  3. “What should we set the air mass coefficient for testing PV cells?” This is the 3rd factor that has to be the same for all tested solar panels, and needs to be uniformly set. It has to deal with the optical path length of light through the atmosphere; this Wikipedia article explains that in a bit more detail.

Basically, when we get 100 different solar panels from different manufacturers, we need to devise a uniform set of test conditions we can produce in the lab that will tell us all the specs we need: solar panel nominal power (Wp), rated power voltage (Vmp), rated current (Imp), open circuit voltage (Voc), short circuit current (Isc), and so on.

These are the STC lab conditions that IEC came up with in 1993 and that we still use today as the primary set of test conditions for solar panels:

Standard Test Conditions (STC)

Measured Quantity: STC Conditions:
Solar Irradiance: 1,000 W/m2 (92.90 W/sq ft)
Cell Temperature: 25°C (77°F)
Air Mass: 1.5

This chart tells us that all those solar panel power ratings, voltages, and currents are measured at:

  • Solar irradiance of 1,000 W/m2. In the real world, we get 0 W/m2 at night and up to about 1,500 W/m2 on a very sunny day without clouds.
  • Cell temperature is held constant at 25°C (77°F).
  • Air mass coefficient is 1.5.

When a manufacturer wants to test their new solar panels, the IEC creates these test conditions in a laboratory, puts the solar panels under that 1000 W/m2 light, and measures the solar panel output.

Here is an example of the specs the STC test gives us:

STC Specifications Example

Here is a full datasheet for SunPower X-Series residential solar panels. You may note that the datasheet starts by listing all the tests and certifications these solar panels have (Standard Tests: UL 1703, Type 2 UL Module Fire Rating, IEC61215, IEC61730, Class C IEC Fire Rating, Quality Tests: ISO 9001:2015, ISO 14001:2015, EHS Compliance, Ammonia Test, Desert Test, Salt-spray Test, PID Test, etc.).

Further on, you can see that SunPower lists platform electrical data at STC conditions, NOCT conditions, and at Low Irradiance conditions. If you check the performance at STC conditions for the SPR-X21-470-COM module, you can see these specs measured at Standard Test Conditions (STC):

Solar Panel Specifications: STC Measured Quantity:
Solar Panel Nominal Power (Wp): 470 Watts At STC
Rated Voltage (Vmp): 77.6 Volts At STC
Rated Current (Imp): 6.06 Amps At STC
Open Circuit Voltage (Voc): 91.5 Volts At STC
Short Circuit Current (Isc): 6.45 Amps At STC

As you can see, whenever looking at solar panel specs, you have to check if the specs were measured at STC, NOCT, or NMOT conditions.

Here’s why this is:

This SunPower SPR-X21-470-COM solar panel has an STC power rating of 470 watts. If you would check the 3rd chart in that datasheet, you might think that the power rating is 356 watts. That’s because, in that 3rd chart, you have a list of specs that were measured at NOCT conditions (key difference is 200 W/m2 lower sun irradiance; NOCT uses 800 W/m2 and STC uses 1,000 W/m2).

You can read more about these STC vs NOCT differences here.

Limitation Of STC Or Why We Need NOCT, NMOT

Clearly, we don’t test solar panels only at STC conditions. In many cases, you will see specs measured at NOCT or NMOT conditions. Why is that?

Well, the main metric we want to know about solar panels is the power rating. As we have seen, this depends on solar irradiance, cell temperature, air mass, and so on.

The key to remember here is that STC conditions may be the golden standard, but in the real world that SunPower SPR-X21-470-COM solar panel will produce closer to 356 Wh of electricity per hour (NOCT conditions) than 470 Wh (STC conditions).

The STC conditions are a bit theoretical. They are very useful if you need to put all the solar panels through the same test. However, for calculating the electricity output, they fall a bit short.

In order to introduce test conditions that work better in practice (give realistic electricity generation), the NOCT or Nominal Operating Cell Temperature specs are used. NOCT additionally accounts for wind speed and uses outdoor air temperature (instead of PV module temperature STC uses) to give a realistic power rating (as well as corresponding voltages and amps).

All in all, the STC is the golden standard for testing solar panels. It might be a bit theoretical, but it’s not going anywhere. If you are researching which solar panel to buy and are trying to figure out how much electricity a specific solar panel will generate, the STC measured specs are a good estimate. NOCT specs are, in practice, a bit more accurate.

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