Wet dreams
Most often, the energy lost in a wet dream is much less as compared to a voluntary ejaculation. Sometimes it is mostly prostatic fluid without much or negligible sperm. A wet dream occurs in dream state/ semi-conscious state when the sub-conscious mind is at work. Hence the wastage is not very high as compared to voluntary ejaculation in the awake state.
However it becomes important to realize that wet dreams are a clear indication of dormant lust unfulfilled. Since the mind has been suppressed from such thoughts while awake, it resorts to them in the dream when the intellect and consciousness are at their low.
This is an indication that the aspirant should work harder towards realizing the fallacy and unreal nature of the senses and enlighten himself with truth. The Brahmachari(celibate) must continue to practice Brahmacharya(celibacy) diligently, understanding that these experiences are part of the journey towards self-realization.