Word for energyless-ness

Solution 1:

Exhaustion: the state of being exhausted.

Where exhausted is the state of having used all of someone's mental or physical energy : to be tired out or worn out (someone) completely; to be completely used up.

Exhaustion has the meaning of being completely "energyless", whereas words like lethargy and fatigue more closely mean "having little energy".

Solution 2:

Depending on the context, "Lethargy" may be a fit.

http://m.dictionary.com/definition/lethargy

Similarly there is: "sluggishness", "listlessness", "lassitude" and "fatigue".

Surely one of these will fit.

UPDATE:

In light of clarifications, FATIGUE is the word you seek.

http://m.dictionary.com/definition/fatigue

Solution 3:

To emphasize the lack of energy, use exhaustion. To emphasize the effort that led to exhaustion, use fatigue. For a more informal term, you can use burnout.

Solution 4:

If someone is losing energy that would imply that they do have some energy remaining. However if you want to convey total exhaustion, you could use the word, "spent".

Solution 5:

drained would be suitable.

to exhaust physically or emotionally < feeling drained at the end of a long workday >