Why a "400 W panel" never actually produces 400 W of continuous real-world output — even on a perfectly clean, perfectly sunny day.
Every solar panel is printed with a nameplate wattage — 400 W, 450 W, whatever the model happens to be — and that number is not a lie. It's measured honestly, in a lab, under a standardized test. The catch is that the standard used to generate that number, Standard Test Conditions (STC), describes a set of conditions a real installed panel almost never experiences once it's bolted to a roof in the actual sun. A second standard, PVUSA Test Conditions (PTC), rates the same panel under conditions much closer to how it actually behaves in the field — and it consistently comes out 10–15% lower. Neither number is wrong. They're measuring the same panel under two deliberately different sets of conditions, and knowing which one you're looking at is the difference between an accurate production estimate and a system that looks "broken" for no reason at all.