Understanding NMOT In Solar: NMOT vs STC vs NOCT Explained

If you check solar panel specs sheets, you will notice some unfamiliar terms. NMOT, along with STC or even NOCT are some of them. Today, we will focus on explaining what NMOT means in solar and how it relates to STC as well as to NOCT.

Let’s first explain what these solar abbreviations mean in broad terms:

  • NMOT in solar stands for Nominal Module Operating Temperature.
  • STC stands for Standard Test Conditions. This is the primary and most basic set of test conditions we use to measure the output of solar panels.
  • NOCT stands for Nominal Operating Cell Temperature.

The reason why we mention these 3 solar abbreviations together is that, on solar panel specs sheets, you can see something like this (for exactly the same solar panel):

  • Solar panel power rating PMax (at STC): 300 Watts.
  • Solar panel rating PMax (at NOCT): 250 Watts.
  • Solar panel power rating Pmax (at NMOT): 230 Watts.

So, what is the correct solar power rating here? 300W? 250W? 230W? The usage of these different power ratings is a bit confusing. Let’s start clearing them out by starting at the beginning:

From STC To NMOT

When solar panels were invented, we needed to create a set of test conditions at which we will measure solar power output. We need to measure power rating or wattage (Pmax), rated voltage (Vmp), rated current (Imp), open circuit voltage (Voc), short circuit current (Isc), and so on.

The first set of basic test conditions, proposed by the International Electrotechnical Commission (IEC) in 1993 and currently covered by this IEC 61215-1:2021 document, are the STC.

STC means that we measure out solar panel output at Standard Test Conditions, which are:

  • Solar irradiance of 1,000 W/m2.
  • Cell temperature is held constant at 25°C (77°F).
  • Air mass coefficient is 1.5.

Now, the STC measurements of output (300 watts in our example above) are useful when we have to denote the solar panel power rating (300W solar panels) and compare different solar panels.

However, the STC are the ideal lab-made conditions that don’t really occur in the real-world (when you put solar panels on the roof). To more precisely account for real-world conditions, the NOCT and later NMOT test conditions were introduced.

Basically, NOCT test conditions use lower 800 W/m2 solar irradiance and measure the outdoor temperature as well as cell temperature instead of measuring just the temperature of the PV module and accounts for 1 m/s wind speed. You can read more about STC itself here and the differences between STC and NOCT here.

What both STC and NOCT test conditions fail to account for is that, under intense solar irradiance, the solar panels heat up. This usually negatively impacts their efficiency.

To account for this increase in solar panel temperature, NMOT was introduced. Compared to NOCT, the NMOT or Nominal Module Operating Temperature test conditions introduces these two key changes:

  1. Instead of measuring cell temperature (like NOCT), NMOT measures the back-of-module temperature.
  2. All measurements are made while the solar panel is under load.

We are going to look at how this effect the power rating we found on solar panel specification sheets. It is important to note that in the 2016 IEC 61215 document, the IEC started using NMOT measurements instead of NOCT measurements.

That’s why you will find:

  • STC and NOCT specs on older (2016 and prior) specification sheets. An example would be this SunPower E-Series solar panels (you can see, for example, nominal solar power Pmax at STC and at NOCT.
  • STC and NMOT specs on newer (2017 and beyond) specification sheets. Here, an example would be these SonnenStrom Fabrik solar panels (you can clearly see the solar panel performance at STC and at NMOT).

Now, here are what those two differences bring, compared to NOCT measurements:

Nominal Module Operating Temperature (NMOT) Specifics

There is a good reason why the NMOT power rating in our example (230W) is lower than the NOCT power rating (250W) and much lower than the STC power rating (300W).

Namely, according to the article by Aaron Wheeler et al. (University Of California) on “Determining the Operating Temperature of Solar Panels on Vehicles”, those two differences between NOCT and NMOT manifest like this:

  • There is a “3°C difference between the back-of-module temperature and the cell temperature for operation in an open-rack configuration.”
  • “The effect of removing 15% to 20% of the energy by driving an external load rather than operating at open-circuit voltage can be estimated to decrease the temperature by an additional 3-5°C.”

As we can see, both the back-of-module temperature and measuring solar panels when under load (not at 0% load or open circuit voltage) will decrease the solar panel power rating.

This makes sense even without looking at scientific temperature:

NMOT measurements account for higher solar panel temperature because solar panels will heat up when you put them on your roof. At higher solar panel temperatures (above 77°F temperature, in general), the efficiency of solar panels drops.

measuring solar panel temperature for nmot output calculation
Technician measuring the solar panel temperature and the resulting efficiency drop (at higher temperatures). This decreases the real power rating (Pmax).

In solar, we measure this drop in efficiency at higher temperatures with a solar panel temperature coefficient. Solar.com made a good analysis; for every additional degree Celsius (+1°C), the efficiency will drop by anywhere between 0.3% and 0.5%.

From the study referenced before, we can see that, compared to NOCT measurements, the NMOT measurements are conducted at 6°C higher temperature.

That means that the efficiency of solar panels drops by 1.8% to 3.0%. If we presume that the average solar panel efficiency is about 20%, this is a 10% decrease in efficiency between NOCT and NMOT.

And the difference in power rating in our example – 250W at NOCT and 230W at NMOT – is right about 10%.

Which One Is More Important: NMOT vs STC vs NOCT?

Now that we understand what NMOT means, we still have the same problem:

Is that solar panel from the initial example 300W, 250W, or 230W?

Here is the easy way of thinking about it:

It is a 300W solar panel. Why? Well, because the governmental authorities and IEC ruled that we will use STC measurements (300W power rating) as the golden standard.

Why do we need NOCT and NMOT then?

When we try to estimate the real solar panel output, it is much more useful to use NOCT specs, and the NMOT specs are even more useful. Especially if your solar panels get a lot of sun (that’s when solar panels make the most sense), checking the NMOT specifications will more accurately give you an idea of how many kWh your system will generate per day, for example.

NMOT test conditions account for the most conditions (solar irradiance, wind speed, air mass, back-of-module temperature, efficiency drop at higher solar panel temperatures, measuring the solar panel output when under load) and will thus produce the most accurate real-world solar panel outputs (in terms of wattage, voltage, and amps).

5 thoughts on “Understanding NMOT In Solar: NMOT vs STC vs NOCT Explained”

  1. Oh wow, so, on a hot summers day in North Wales, I guess the panel temperatures could reach 40’C, reducing their output by (40-25)*0.4%=6%, so a 343watt (NMOT) panel would actually produce 94% of that, i.e 322 watts?

    Reply
    • Hi Mike, yes, that’s the gist of it. The temperature difference is from 25°C to 40°C is 15°C. If the temperature coefficient is -0.4% (rough average), you will lose about 6% of the output because the panels are hotter than 25°C, bringing the real-time capacity from 343W to 322W. Your math is on point, great job.

      Reply
  2. This is super helpful was comparing Jinko and REC Solar pannels and Jinko was ahead based on STC wasn’t until i compare NMOT and then fine print of there NOCT that i realised in Australia’s hot climate i would never achieve top output.

    Reply
  3. So which should be used for calculations then regards to inverter safety, which standard should be taken into account when looking at inverters max DC input voltage?

    Reply
    • For determiening the max DC input voltage, you only have to look at the Voc (Open Circuit Voltage). This is the highest voltage in your panel system; the inverter sizing is based on Voc.

      Reply

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