What the 1500 W peak pulse rating means for your protection design
The SMCJ40A-13: For a 40 V nominal rail, the reverse standoff voltage is 40 V, so it sits across the line and stays transparent until a surge pushes the voltage above the 44.4 V minimum breakdown threshold. That clamp voltage tells you the downstream silicon must survive at least 64.5 V for the duration of the surge — a 60 V-rated MOSFET or 63 V capacitor is marginal here; 75 V or 100 V parts give proper margin.
Package, temperature, and board-fit
The SMC footprint is larger than the smaller SMA/SMB packages, which helps spread the transient heat into the PCB copper — important for repetitive surge events where the junction temperature accumulates. No power line protection feature is claimed — this is a signal- or low-power-rail TVS, not a mains-grade protector.