Word for 'energy-less-ness'

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".

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

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.

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".

drained would be suitable.

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