What is the antonym of energetic?

The antonym of energetic depends on the context, but some common ones are:

General:

* Sluggish

* Lethargic

* Tired

* Exhausted

* Inactive

* Lazy

* Passive

* Weak

Specific to a particular type of energy:

* Dull (for mental energy)

* Quiet (for physical energy)

* Calm (for emotional energy)

* Depressed (for mental energy)

Formal:

* Enervated

* Debilitated

* Listless

The best antonym will depend on the specific situation and what kind of energy you are referring to.