Sunny July noon and my 440 W modules only make about 390 W each. Are they faulty?

The short answer

Probably not. At −0.29%/°C, a 440 W module with 65 °C cells makes about 389 W at full sun, by the datasheet coefficient.

Plain explanation

Rated power is measured at STC: 1000 W/m² and 25 °C cell temperature. On a roof in summer the cells run much hotter. The JinkoSolar JKM440N-54HL4R-B datasheet gives a Pmax temperature coefficient of −0.29%/°C, so 40 °C above STC costs about 11.6%: 440 W × (1 − 0.0029 × 40) ≈ 389 W. The same datasheet lists 331 W at NOCT (800 W/m², 20 °C air, 1 m/s wind). The 65 °C cell temperature here is an illustration, not a measurement; real cell temperature depends on irradiance, wind and mounting.

Steps from the source

Compare against irradiance and module temperature, not against the label
Check the Pmax temperature coefficient when choosing modules (closer to 0 is better)
Keep ventilation under the modules (see qs-112)

"Temperature coefficients of Pmax -0.29%/°C"

Word for word from the source.

Source

JinkoSolar JKM420-440N-54HL4R-B datasheet (F1.3-EN), p. 2 electrical table: 440 Wp STC / 331 Wp NOCT; 'Temperature coefficients of Pmax -0.29%/°C'; NOCT 45±2 °C; STC = 1000 W/m², 25 °C cell, AM1.5www.jinkosolar.com ↗

Where installers ask this

General guidance based on the manufacturer's manual or the named source. Always follow the current manual for your exact model and your national code. DC and AC electrical work is for qualified installers only. Not affiliated with or endorsed by any brand named; trademarks belong to their owners.