Understanding solar panel output: Standard Test Conditions vs. Real world

Calculating the size of your future solar system is tricky. The real-world power output of a solar panel never matches its nameplate. Where do the numbers come from then? In this article, we’ll talk about Standard Test Conditions (STC) and Nominal Operating Cell Temperature (NOCT). You’ll learn what you can expect from your panels and with that, understand the process of planning a solar system better.

What are Standard Test Conditions? Lab numbers

The most important characteristic of a solar panel is its power output. You can find it in the panel’s spec sheet. Power represents voltage multiplied by current and is measured in a lab when the panel is tested. The power in the spec sheet is what the panel shows at Standard Test Conditions or STC.

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STC represents a set of controlled laboratory conditions under which manufacturers measure a panel’s performance. STC establishes a crucial common ground for testing solar panels across the whole solar industry. This allows to compare different solar panels apples-to-apples ensuring fair competition between panels from different manufacturers.

STC encompasses three key parameters that define the controlled environment. These are irradiance, cell temperature, and air mass.

Irradiance

Measured in watts per square meter, irradiance represents the intensity of sunlight falling on the solar panel. STC standard specifies an irradiance level of 1000 W/m². This value approximates the intensity of direct sunlight at noon on a clear day.

Cell temperature

Solar cells generate electricity through the photovoltaic effect, which is more efficient at cooler temperatures. STC standard dictates a cell temperature of 25 C or 77 F. This temperature reflects ideal operating conditions for solar panels.

Air mass

Air mass refers to the path length that sunlight travels through the atmosphere before reaching the Earth’s surface. The more atmosphere, the more sunlight that is refracted or absorbed by air and dust. STC standard specifies an air mass of 1.5. This value approximates the average air mass for most locations on Earth.

These parameters create an ideal environment for maximum solar panel’s performance – no shade, no cloud, no wind. The amount of power a solar panel generates under the Standard Testing Conditions becomes its maximum power rating or nameplate capacity. If a solar panel outputs 400 watts at STC, it will be labeled as a 400-watt solar panel.

Unfortunately, your solar panels will rarely, if ever, experience these Standard Test Conditions. Lab conditions hardly represent the real world with uneven irradiance, clouds, rain and snow. This is why alongside STC performance, some manufacturers mention the expected output of a panel under Nominal Operating Cell Temperature or NOCT in the datasheet. 

What is Nominal Operating Cell Temperature? “Real” world

Nominal Operating Cell Temperature (NOCT) or Nominal Module Operating Temperature (NMOT) provides a more realistic picture of a solar panel’s performance in actual operating conditions. NOCT considers an air temperature — not cell temperature! — of 20 C or 68 F, solar irradiance of 800 W/m2, a wind speed of 1 m/s, and open backside mounting. 

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Some manufacturers specify the performance of their panels under NOCT conditions in the datasheets. You can immediately see how much a panel is likely to generate on your roof. As you see, your panel will produce about 70-80% of its maximum power on a sunny day. For a 400-watt solar panel, it’s about 300 watts. This is why when calculating your system’s capacity, it is so important to consider NOCT alongside STC ratings or leave a 30% margin if NOCT power is not directly in the spec. 

Among manufacturers specifying the panel characteristics at NOCT are Q Cells, Jinko, Meyer Burger, Phono Solar, Solaria, REC, Silfab, ZNShine Solar, and some others. 

Out of stock

Jinko 390W Solar Panel 144 Cells JKM390M-72HBL-V Clearance Assembled in USA

• Price per Watt$0.46
• Rated Power Output 390 W
• Voltage (VOC)48.6V
• Number of cells144
• Cell TypeMonocrystalline

Delivery on Dec 31 – Jan 06

NOCT also has a more narrow meaning. It represents the temperature reached by cells in a module under the above conditions, expressed in degrees Celsius. When working, solar panels always get hotter than the air. If it’s 20 C outside, the temperature of a PV module may reach 45 C. You can read about it in more detail in our article “Too much sun: What is the temperature coefficient of solar panels”.

Julia Zaraeva

Author

Years of experience in translation and a love of nature help Julia find the right words to encourage going solar. She joined the team in 2023 and is happy to make her contribution to a greener future.

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