Manufacturers test different conditions of solar panels to rate them. But the question is which rating is important to consider. Some of these ratings include STC - Standard Test Conditions refers to measuring in lab conditions. The PTC-Photovoltaic USA refers to energy ratings The rating basically works through several variables to evaluate the functions of solar panels in real-world conditions. For example, STC evaluates solar panels in the solar simulator which is commonly called flash tester. During this testing, testers evaluate panels in the presence of artificial sunlight that simulates specific conditions in which panel squarely faces the sun. Some of the standard testing conditions are; Temperature = 25°C / 77 degrees F Air Mass = 1, 5 AM (Air Mass, it measures air thickness in the atmosphere. (Equator=1 AM Solar Irradiance =1000 W/m² The PTC includes a few extra variables when the tester measures the solar panel level. It still uses 1,000 W/PM variables and 1.5 air mass. Besides this, it also sets temperature that goes up to 20°C, when positioned at 33 ft above the ground level, with a wind speed of 1.5 meters. Remember that solar panel rating is quite helpful when it comes to evaluating the panel options. It can help you decide which solar panel will perform better and generate energy.