1500 W peak pulse, 12.8 V standoff — what the ratings mean
The 1.5SMC15: That 1500 W rating is the energy it can clamp once — repeated surges at that level will exceed the junction's thermal capacity, so derate for repetitive events. The 12.8 V standoff means it sits across a 12 V rail without conducting leakage; the 22.26 V clamp is where it holds the surge — downstream circuitry must survive that peak.
Obsolete — sourcing through surplus channels
Littelfuse lists the 1.5SMC15 as Obsolete. This part is available through independent surplus and broker channels — quantities are lot-specific, date-code spread and packaging condition (sealed reel vs cut tape) confirmed at RFQ. No pin-compatible direct replacement is documented; a board spin would be required to use a different TVS family.