Solar Panel manufacturers use what is called Standard Test Conditions (STC). This means they put the solar panels in a flash tester in their factory that has been calibrated to deliver the equivalent of 1000 watts per square meter of sunlight intensity, hold a cell temperature of 25'C (77'F), and assume an airmass of 1.5. This flash test gives them their STC ratings. Consider these as you would EPA mileage ratings ... how many of you actually get the mileage they post?