The label says 440 W. The July roof says about 389 W.

The short answer

Not broken. Just hot: at −0.29%/°C, cells at 65 °C lose around 11.6%.

Plain English

Label: the 440 W on a JinkoSolar JKM440N-54HL4R-B is STC power, measured at 1000 W/m² and 25 °C cell temperature.

Datasheet: Pmax temperature coefficient −0.29%/°C; 331 W at NOCT (800 W/m², 20 °C air, 1 m/s wind).

Worked example: 440 W × (1 − 0.0029 × 40) ≈ 389 W with cells at 65 °C. That is a calculation from the datasheet coefficient at an example cell temperature, not a measurement.

Real cell temperature depends on irradiance, wind and mounting, so compare output against irradiance and module temperature, not against the label.

Installer tip: keep the ventilation gap under the modules (JinkoSolar asks for at least 10 cm) and check the Pmax coefficient when choosing modules; closer to 0 is better.

Source

JinkoSolar JKM420-440N-54HL4R-B datasheet F1.3-EN (p. 2: 440 Wp STC, 331 Wp NOCT, Pmax −0.29%/°C, NOCT 45±2 °C)www.jinkosolar.com ↗

JinkoSolar Global Installation Manual 202605 A1.8-EN (PDF p. 10: ventilation gap of at least 10 cm)jinkosolarcdn.shwebspace.com ↗

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The full answer

How many watts do your modules really make on the hottest day?

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