STC vs NOCT: Understanding Test Conditions For Solar Panels

What’s the difference between STC and NOCT? When it comes to solar panels, this question comes up quite a lot. For estimating solar panel output, wire sizes, breaker sizes, max. power, voltages, and so on, we have to know if we should use STC or NOCT conditions.

Distinguishing between these two solar panel test conditions will help us plan out our solar system and, what is even more important, give us an insight into how to compare solar panels with the same STC but different NOCT (during buying process).

Here is what both of these abbreviations stand for in the context of solar panel testing conditions:

  • STC stands for Standard Test Conditions. All solar panel manufacturers have to measure wattage under these conditions. Example: We can say that a 300-watt solar panel is, in fact, a 300-watt solar panel because it produces 300 watts of electricity under STC conditions. These are the lab-created perfect condition; full sunshine, no clouds, no wind, and low temperature to prevent cell overheating (and decrease in efficiency).
  • NOCT stands for Nominal Operating Cell Temperature. STC is more of a theoretical solar panel wattage measurement. In the real world, the NOCT gives you a more practical insight into how much electricity a solar panel actually produces. These don’t presume peak sunlight (some clouds) but do presume higher air temperature and wind.

Example of STC and NOCT application: Two different solar panels can have the same STC specifications. They have the same performance, right? Not exactly. If we were to compare NOCT specs (some specs sheets include those), we would see that the real-time performance is different, and we should pick a solar panel with the same STC specs but better NOCT specs.

Case and point: Further on, we compared 2 solar panels (SunPower vs REC Peak Energy) with the same 310W rated power under STC conditions. To figure out which is better (produces more electricity), we looked into real-world NOCT specs. Here we see that the power these 310W-rated panels produce is 235W and 225W, respectively. We will look at the full STC and NOCT specs comparison further on.

stc vs noct solar panels specifications comparison
STC are the perfect conditions (no clouds, high 1000W/m2 irradiance) and we have a higher rated power than NOCT. NOCT are real-world conditions (some clouds, lower 800 W/m2 irradiance) and we have a lower rated power than STC.

To fully understand STC vs. NOCT differences and when to use STC vs. when to use NOCT, let’s first look at each of these solar panel conditions in turn. Let’s start with STC:

STC Or Standard Test Conditions

When solar panel producers have to tell how much electricity a solar panel produces, they have to use the same set of conditions to measure the wattage, voltage, amps, and so on. The agreed test conditions all manufacturers have to adhere to are called Standard Test Conditions (STC) and are as follows:

  • Irradiance: 1000 W/m2. For homeowners from the US, UK, and all other countries that use feet instead of meters, this is equal to 92.90 W/sq ft.
  • Cell temperature: 25°C (77°F).
  • Air mass: 1.5.

As we can see, the STC conditions will measure the wattage, Voc, Isc, and so on at laboratory-precise conditions that include irradiance (how much sun shines on the panels), the temperature of the PV cells (note that this is not outdoor temperature, but the temperature of the cell itself), and air mass (you can read more about the relevance of air mass for solar here, it has to do with the angle of the sun or azimuth).

When you are comparing different solar panels, you are basically looking at specification sheets, right? Looking at wattage, Voc, Isc, and so on. All the figures on the spec sheets are measured under STC precisely so that we can compare different solar panels.

However, the STC are basically strict lab conditions. In practice (when you put solar panels on your roof, for example), you will notice that:

  • Irradiance is not 1000 W/m2. At night, it will be very close to 0 W/m2, and, during a scorching day, it can be above 1000 W/m2.
  • Cell temperature will not remain at 25°C (77°F) all the time. In fact, the temperature within a PV cell can be about 20 to 30 degrees higher than the outdoor temperature. The temperature in the lab they measure this in is usually between 0°C and 5°C; this keeps the cell temperature at 25°C.
  • Air mass is not constant at 1.5 but changes with the angle of the sun (hour of the day).

We see that there is a fair discrepancy between the theoretical STC conditions they measure the solar panel specs at, and the real-time conditions.

To alleviate that – and give us the best information possible on the performance of solar panels in practice – a different set of test conditions was introduced in 1980 by Harvard scientist R. G. Ross Jr. and dubbed NOCT. The goal of NOCT or Nominal Operating Cell Temperature conditions was to estimate as closely as possible the real specifications of solar panels in practice:

NOCT Or Nominal Operating Cell Temperature

It turns out that solar panels when installed don’t really match the figures on the specs sheet. That’s because specs are measured at STC conditions, and these conditions don’t exactly relate to real-world conditions. That’s why we use NOCT conditions to estimate the solar panel output, for example.

Compared to STC, NOCT is another set of conditions that are more closely related to real-time conditions, and are as follows:

  • Irradiance: 800 W/m2. That’s equal to 74.32 W/sq ft. NOCT uses 200 W/m2 lower irradiance than STC.
  • Air mass: 1.5. This is the same for NOCT and STC; no difference here.
  • Air temperature: 20°C (68°F). This is not a 5°C difference between STC and NOCT; it’s a completely different temperature. While STC measured the temperature of the PV cell, NOCT measured the outdoor air temperature.
  • Wind speed: 1 m/s (this is equal to 2.24 miles per hour wind). NOCT includes this extra parameter that STC doesn’t account for.

Due to these different test condition parameters, there can be quite a big difference between STC and NOCT specifications.

Now, why does all that matter? Well, it matters because everybody wants to use the best solar panels. The way to tell which solar panels are the best is by comparing these specs.

In order to fully understand the usefulness here, let’s look at STC vs NOCT specifications in use:

STC Vs NOCT Specification Comparison

Alright, let’s have a look at two solar panels that have the same STC Pmax of 310 watts. We are going to compare the SCT vs NOCT specifications of these two solar panels:

  1. SunPower E-Series SPR-E19-310-COM with 310W nominal power. Here is the SunPower E-Series data sheet.
  2. REC Peak Energy 72 Series REC310PE72 with 310W nominal power. Here is the REC data sheet.

With both of these panels, we see that the data sheet specifies STC and NOCT specs. Here is an example of the SunPower solar panel:

SunPower 310W Panel: STC Specification: NOCT Specification:
Nominal Power (Wp): 310 Watts 235 Watts
Rated Power Voltage (Vmp): 54.7 Volts 51.5 Volts
Rated Current (Imp): 5.67 Amps 4.67 Amps
Open Circuit Voltage (Voc): 64.4 Volts 51.5 Volts
Short Circuit Current (Isc): 6.05 Amps 4.89 Amps

As we can see, the SunPower panel does have a rated nominal power of 310 watts under STC conditions. However, under the real-time NOCT specifications, we have a 235 watts nominal power. That means that in practice, this SunPower solar panel will likely produce 75.8% of its specificated power.

We also see that voltages and currents (not only wattage) are different when comparing STC and NOCT specs. NOCT has higher Vmp, Imp, Voc, and Isc than STC.

Now, let’s look at the same specifications (STC vs NOCT) for the 2nd REC Peak Energy 310W panel:

REC Peak Energy 310W Panel: STC Specification: NOCT Specification:
Nominal Power (Wp): 310 Watts 225 Watts
Rated Power Voltage (Vmp): 36.7 Volts 30.4 Volts
Rated Current (Imp): 8.53 Amps 7.41 Amps
Open Circuit Voltage (Voc): 45.3 Volts 37.4 Volts
Short Circuit Current (Isc): 9.02 Amps 7.77 Amps

With this solar panel, we see the same STC power of 310W. Checking Vmp, Imp, Voc, and Isc under STC conditions, we see that REC uses lower voltages and higher currents than SunPower. Example: SunPower has 6.05 amps short circuit current (Isc), while REC’s panel has a higher 9.02 amps current. This higher current is offset by using lower voltages, in order for both panels to have the same rated power – 310W.

Looking at voltages and currents is interesting, but what really matters is the nominal power (Wp). We see that REC Peak Energy solar panel is rated 310 watts under STC, but will produce 225 watts under NOCT conditions. That means it produces 72.6% of its specified power output.

This is a clear situation when we can apply the STC vs NOCT difference to figure out which solar panel – SunPower or REC Peak Energy – is better. Namely, both have the same 310W nominal power under STC conditions. They have the same performance, right?

Not really. What we want to compare is the NOCT real-time specs, and here we see that:

  • SunPower panels actually produce 235W or 75.8% of the rated STC power.
  • REC Peak Energy panels actually produce 225W or 72.6% of the rated STC power.

Here we see a clear 10-watt difference between the two solar panels under NOCT conditions (in favor of SunPower). That means that SunPower 310W panel will produce about 40Wh per day (or 14.6 kWh per year) more electricity than REC Peak Energy solar panels that has the same STC specs (calculation presumes a 12-month average of 5 peak sun hours and account for 25% solar system losses).

If we were to construct a 5kW system with SunPower panels (18 310W panels), this system could produce 262.8 kWh of electricity more than the 5kW system made out of REC Peak Energy solar panels, despite the same STC-rated power. The key here are the NOCT specifications.

Ask A Question, Share Your Story

We hope that this illustrates well the differences between STC and NOCT datasheets, and, most importantly, how to use predominantly the NOCT data when comparing solar panels.

If you have any questions or would like to share a story about your solar system or panels, you are most welcome to use the comment section below.

5 thoughts on “STC vs NOCT: Understanding Test Conditions For Solar Panels”

  1. thank you very much i got a lot of info, also i need some software that can help me to design solar for big projects even i will pay for it

  2. On a practical note, tracking recorded peak production vs. STC, I’m seeing numbers more like 81-82% of STC in the hot sun of Texas (mostly older panels). I’m guessing the irradiance is higher than STC during the summer months, though cell temperatures are much higher than NOTC. In talking with several installers, they tend to use 80-85% of STC to calculate the practical peak production for design purposes (i.e. inverter max PV power in). If course it depends on the brand of inverter and whether or not this derating is already taken into consideration. Thanks for your article.

    • Hi Ed, thank you for your input. 81-82% would be in line with what you would theoretically expect in Texas, California. For kWh production, we usually take STC and reduce the overall electricity production by 25% (so, 75% of the total). That accounts for losses in the system but also offsets the STC conditions a bit.

  3. Thankyou for taking the time to write this – I found it really useful : ) Still trying to work out whether to use NOCT or STC for voltage/current calculations and it seems NOCT is much more sensible given that STC seems to be ideal conditions. (Although designing a system around STC would therefore seem to include some ‘overhead’ in terms of cable ratings/fuses etc. Thanks again…


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