Can I activate an enchantment energy ability during an opponents turn?
You can activate an activated ability at any time that you could cast an instant. This includes on your opponents' turns. Being an energy-related ability has no bearing on this.

There are several different times throughout a turn that each player receives priority, which is when that player can act. In terms of combat, you could activate it either when the combat phase begins, or after your opponent declares his attackers, or after you declare your blockers.