Energy is used rather than power as it's the cumulative effect in energy incidents that causes damage. In most of the world, the unit is J/cm2, though.

Ah, all right, I understand if it's used to rate accumulated damage for incidents. For ongoing arcs, or comparing them however, I think it's a weird.

A small arc, not even dangerous to touch, could have the same energy rating as a gigantic one if you measure the first over a year and the second over a millisecond.