Elemental Fury (Ex): At 3rd level, whenever the elemental kin takes an amount of energy damage equal to or greater than her barbarian level while raging, she adds 1 to the total number of rounds that she can rage that day. At 6th level, and every three levels thereafter, the number of extra rounds per energy attack increases by +1, to a maximum of +6 rounds per energy attack at at 18th level. This ability replaces trap sense.
Question is, what constitutes 'energy damage'?
Fire, acid, cold, and electricity are no-brainers, of course.
Sonic might count (though it doesn't exactly fit the 'elemental' type)...
However, what about Force? Or, negative Energy?
Bonus question: what is the time interval that counts for 'taking energy damage equal to or greater than her barbarian level'?
One attack? (so, no effect if I get hit by two flaming attacks that both are below the threshold)
One round?
Per rage (which would mean some serious bookkeeping)?
Question is, what constitutes 'energy damage'?
I would assume any energy type that can be resisted against via resist energy or protected against via protection from energy (acid, cold, electricity, fire, and sonic). However I could not find anything referencing "energy type" in the PRD's glossary.
what is the time interval that counts for 'taking energy damage equal to or greater than her barbarian level'?
From a single source at one time. If you get hit with two seperate flaming attacks that are both below your barbaian level in one round you do not get to add them together. This means damage over time effects like acid arrow, which do small amounts of damage for longer periods, are unlikely to trigger your elemental fury.
In summary; when you take acid, cold, electricity, fire, or sonic damage, check to see if that damage is greater than or equal to your barbarian level. If it is, add the extra +1 round to your total rage rounds (or more depending on your level). There is no need for further math or book-keeping than that.