"400W" is a laboratory number

Panel marketing leans hard on one number that doesn't describe your roof.

"400W" is measured under Standard Test Conditions — 1000 W/m² irradiance, 25°C cell temperature. That's a laboratory, not a summer afternoon.

The numbers that actually matter:

Temperature coefficient — how much output drops as the panel heats. This is why the same panel produces more on a cold clear day than a hot one, and why airflow under the panel is worth designing for.

Voc (open-circuit voltage) — maximum voltage with no load. Critical for controller sizing, because Voc rises as temperature falls. A cold morning can push string voltage past your controller's maximum and destroy it. Size for the coldest temperature you'll see, not the average.

Vmp and Imp — voltage and current at maximum power. Where an MPPT controller works, and what actually determines harvest.

Efficiency % — matters mostly when space is limited. More watts per square foot. Doesn't make the watts "better," just denser.

The realistic derate: assume roughly 75% of nameplate after heat, wiring, dirt, angle and controller losses.