Sephok

From what I understand repairing anything will return 100% of the power to the void.
Toggling abilities (Nature T1 spearmen, Frost T1 imperials) return 100% to the void as well.
 
When units/buildings die, spells are cast or active abilities are used 90% of the power cost are returned to the void.
 
You get NO power back when a monument or power well is destroyed. Losing 500 power worth of units and casting two spells for 300 power permanently costs you 80 energy. Losing a single power well costs 100 instead, unless there was less than 100 power left in it. You should rarely sacrifice wells or monuments. Splurging a bit and putting two towers down can save you a lot of power later on.
However, I think when you are at T4 and a monument is destroyed you can rebuild it for 100 energy instead of e.g. 300, so it's not quite as horrible as one could first expect (also, that is probably not limited to T4).
 
The game keeps track of how you summoned units: If you use the nature building that reduces summoning cost by 12/15/whatever %, the unit summoned this way will also return less power to the void upon death.
 
Since units dying in battle return the same amount of power as those you kill with the sacrifice button, there are only a few real reasons to kill them yourself instead of sacrificing them in battle:
- you need the void power NOW
- you need the pop space
- the units would actually hinder you (e.g. enemy is shadow and would love to feast on your weak units' corpses)