The power-handling capability of most Zener diodes is rated at 25 degrees C or approximately room temperature. If the temperature is increased, the power handling capability is:

Semiconductor devices, including Zener diodes, are highly sensitive to heat. Their maximum power ratings are typically specified at a standard room temperature of 25 degrees Celsius. As the operating temperature of the diode increases beyond this baseline, its ability to safely dissipate excess heat to its surroundings diminishes. To prevent thermal runaway and the physical destruction of the component, the manufacturer's power rating must be 'derated'. Therefore, the actual power-handling capability is progressively less at higher temperatures than it is at room temperature.