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A Quick Look at STC vs. PTC Ratings

Solar Panels/
A Quick Look at STC vs. PTC Ratings

When going solar, homeowners are often overwhelmed by all the solar panels to choose from. Comparing every option can be confusing, especially if you don’t understand the different rating systems for solar panels: STC and PTC ratings.

This article will shed some light on solar panel STC and PTC ratings to help you through the process of choosing the right panels for your solar energy system.

What are Solar Panel STC and PTC Ratings?

STC and PTC ratings demonstrate how the solar modules can be expected to perform under different conditions.

STC Rating

STC stands for “Standard Test Conditions.” All solar panels are tested under specific criteria of test conditions. You can find the rating on the solar panel model’s datasheet.

The standard test conditions are 1,000 watts per square meter of solar irradiance, 25°C cell temperature and 1.5 air mass.

PTC Rating

PTC stands for “PVUSA Test Conditions.” PVUSA is an acronym for “Photovoltaics for Utility Scale Applications.”

The PVUSA test conditions were developed to demonstrate solar panel performance in more real-world situations than the STC. The PVUSA test conditions are 1,000 watts per square meter of solar irradiance, 20°C air temperature and wind speed of one meter per second at 10 meters above ground level.

Example of a PTC rating on a solar panel specification sheet
Example of a PTC rating on a solar panel specification sheet

Solar Cell Temperature vs. Air Temperature

One thing to note about the difference between STC and PTC ratings is the cell and air temperature variance.

When solar cells operate in full sunlight, their temperature is much higher than in the air. Solar cell temperature is crucial; the warmer the solar cells get, the less power they output.

You can determine the actual cell temperature with a formula based on the solar panel’s Nominal Operating Cell Temperature (NOCT). Once you know the temperature, you can calculate the expected power loss using the temperature coefficient for the solar panel.

For our discussion, know that STC ratings are based on a cell temperature of 25°C while PTC ratings are based on conditions that would create a cell temperature around 45°C — much closer to real-world operating conditions.

Read more: The Impact of Hail on Solar Panels

Why Do STC and PTC Ratings Matter?

STC ratings are significant because the nominal power output of the solar panel is the output in standard test conditions. When we refer to a 400-watt solar panel or a 370-watt solar panel, those wattages are based on STC.

PTC ratings are helpful for calculating the expected output of a solar system in real-world conditions. For example, the California Energy Commission uses PTC ratings to determine solar system rebates.

Evaluating STC and PTC Ratings When Shopping for Solar Panels

PTC ratings for power output are typically 10 to 15% lower than STC ratings. So as you shop for solar panels, draw equal comparisons. Evaluate all solar panels strictly by STC or PTC ratings to ensure you compare apples to apples.

As the numbers suggest, solar panels with a higher PTC rating typically perform better. However, keep in mind there are other factors to consider (besides a high PTC rating) that may contribute to a higher cost per watt.

Learn more: Solar Panel Testing Standards Explained

Our Conclusion On STC and PTC Ratings

Looking at STC and PTC ratings is essential to your solar panel decision but shouldn’t be the only factor you consider. Don’t forget to evaluate solar panels by cost per watt, physical size or aesthetics to help you make the final call.

Learn more about how to choose a solar panel system here. Or, if you need more support selecting the right solar panels for your home or business, connect with the experts at GoGreenSolar. We can help you choose, design and install your solar panel system with ease!

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