All power weapons do Energy damage when there power field is activated. This makes sense, but a lot of the Energy critical damage effects don't really fit being hit by an axe or sword.
So I play that power weapons count as energy damage for all rules effects (such as Throgel's resistance in PtU), but use the rending Critical tables.
On a side note. If a full trained (15000exp) Moritat assassin armed with the Luminous Reproach (from PtU) was attacked by 50 bloodletters, how many demons would the assassin take down before dieing?