The 1N6276ARL4G: Unlike a steady-state power rating, a TVS diode is selected by its peak pulse power versus the expected transient waveform; the 1.5 kW figure applies to the 10/1000 µs waveform used in datasheet characterization.
Standoff, breakdown, and the clamping ceiling
The reverse standoff of 13.6 V typical is the voltage the Zener sees in normal operation without conducting. When a transient drives it above the 15.2 V minimum breakdown, it enters conduction and clamps the voltage to a maximum of 22.5 V at the peak pulse current — that 22.5 V ceiling is the protected rail limit downstream of this device.