As a homeowner, solar panels are a great investment that will yield significant returns for years to come. However, making the right choice in panels can be challenging, as there are many different specifications, including STC and PTC ratings. Knowing the differences will help you better understand how specific panels perform and what you're investing in.

STC

Short for "Standard Test Conditions," the STC rating measures the power output of a panel after a sunlight-like light is flashed onto it. This happens under specific conditions. Every time, 1,000 watts per square meter are shined on the product in a room that's 77 degrees. This refers to both the temperature that's on the cells and the ambient temperature of the room. There are no other variables added to this test. 

Knowing the STC rating is useful when comparing different solar panels to each other because the environment will always be the same. If there's a panel with an STC rating of 150 watts and another at 370 watts, that means that's the output at 77 degrees. 

PTC

As for the PTC, it stands for "Photovoltaics for Utility Scale Applications Test Conditions." Sometimes, this is partially abbreviated to "PVUSA Test Conditions." Unlike the STC, PTC does take into account other variables, and it tests the solar panels as if they were on the rooftop, slowly being heated by sunlight. This means that the temperature on the solar panel cells will go up to about 113 degrees, and it also accounts for an ambient temperature of 68 degrees. Even wind conditions are added to simulate a breeze. 

Under these conditions, the output tends to go down a little bit—about 15% of the STC rating—because when solar panels get hotter, they generate less energy. So, the previously mentioned STC-rated 150 watts panel would likely have a PTC rating of about 127 watts instead.

Which to Use

Because of this discrepancy, some people are quick to consider the PTC to be the more "realistic" of the two ratings. However, since every region is different and conditions are stagnant, both ratings are helpful when researching your solar panel options. 

If you're ready to set up solar panels on your rooftop, get in touch with Olson Solar Energy. Located in Onalaska, WI, this family-owned and -operated company has been trusted throughout the region, in Eau Claire, Stevens Point, Wausau, and beyond. You can see some of their previous projects on their website, and if you'd like to speak to someone directly, call (608) 780-2347. They offer free evaluations, too, so don't hesitate to get in touch.