The behaviour Banshee - Cloaking Field gives the energy loss.
The field...
Modification - Vital Regeneration Bonus (0|0|-1.4765)
is where it is.
Basically it works by giving you negative energy generation. This is good for rather fixed regeneration rates. However for heroes you may want something % based so it scales better. In that case make the behaviour execute a periodic effect that is a unit modification effect that lowers energy state by a percentage.
The game executes 16 frames per game second so if you make a behaviour run an effect that fast it will probably appear smooth enough.