It seems that when choosing ESD/TVS diodes, MOVS and GDTs that while their clamping voltage is rated in Volts, their protection is rated in Watts or Joules.
For example, the LittleFuse SMCJ250CA is 405V Clamping and 3.7A Current. On the front page it states "1500W peak pulse power capability at 10/1000μs".
This makes sense I suppose.
But then the standards I have been reading only talk about transient/surge voltages. For example: 1kV Surge at 1,2us/50us (8/20).
Since the standards I have read are only referencing voltage not current or power. How do I use this information to select a protection device which talks about Watts? Is there some standard current values to assume?