Why do my panels never reach their rated power?

Because the nameplate rating is measured in lab conditions your roof almost never reproduces: 25°C cell temperature, 1000 W/m² hitting the panel dead-on. In real life, cell temperature, sun angle, air clarity and inverter conversion each trim a few percent — so a 6 kWp array peaking at 5.0–5.4 kW on a good day is behaving exactly as designed.

Why this happens

Standard Test Conditions are a benchmark for comparing products, not a promise of daily performance. On a real roof, several deductions stack: cells run hot the moment the sun is strong (losing 0.3–0.4% per degree above 25°C — see the spring-vs-summer answer); irradiance only approaches 1000 W/m² near solar noon on very clear days; the sun meets a fixed roof at an angle most of the time; and wiring plus inverter conversion take their few percent. Systems with a DC/AC ratio above 1.0 additionally clip the rare perfect peaks.

Multiply the factors and a routine best of 80–90% of nameplate is the expected outcome, not underperformance. Ironically, the moments panels do flirt with nameplate are cold, bright spring days — sometimes even briefly above it when sunlight bounces off passing clouds (cloud-edge effect).

What to do

1. Judge daily energy (kWh), not the instantaneous peak (kW) — yield is what pays the bills, and it can be excellent without ever touching nameplate.
2. Compare with a physics-based forecast for your setup — it models temperature, angles and clipping, turning “less than the sticker” into a concrete healthy/unhealthy verdict.
3. Look for asymmetries, not for missing watts — one string persistently below its twin, a lopsided daily curve, or a sudden step down are fault signatures worth an installer visit; a smooth curve 15% under nameplate is just physics.