Energy is a non-physical substance within humans that allows them to sustain physical and mental activity.
Jadeite was first assigned to collect energy for the Dark Kingdom. He usually looked for places where he believed he could collect energy from a large quantity of people. However, once the youma assigned to the task was defeated, usually the energy collected would vanish from his possession. After his death, Nephrite was assigned to collect energy. However, unlike Jadeite, Nephrite tried to collect energy from a single human, after sending a youma to raise the human's energy by causing their interests to become obsessions, and once their energy reached its peak, the youma would take form and absorb the human of energy. Like his predecessor, once Nephrite's youma were destroyed, the energy they stole would be restored to the victim. Queen Beryl used the energy collected to awaken (though not completely revive) Metaria. After her awakening, the Dark Kingdom shifted its goals from collecting energy to retrieving the Silver Crystal.
Shortly after the Dark Kingdom was defeated, two aliens, Ail and An arrived on earth, bringing the Makaiju with them. In order to sustain the tree's life (and therefore their own), the aliens sent Cardians to collect energy.
The Death Busters considered pure hearts to be powerful source of energy as Sailor Pluto confirmed in episode 114. Shortly after the Holy Grail appeared, the Mistress of Silence/Mistress Nine begged pure hearts from people to complete her awakening in Hotaru Tomoe's body. The witches 5 all failed in this task but Kaolinite captured Sailor Chibi Moon and her pure heart was shallowed by mistress nine.
In the Sailor Moon SuperS movie, the main antagonist, Queen Badiane, collected the dream energy of children in order to power up her Black Dream Hole. She did this by placing the children in Dream Coffins, where they were intended to sleep forever and thus continually produce dream energy, although she was also able to offer Chibiusa's energy to the Black Dream Hole directly. This power was called "Sugar Energy."
The Shitennou would collect energy for the Dark Kingdom. This energy was used to awaken Queen Metaria.
- In the original Japanese version of the anime, "energy" is pronounced enerugi like the German word "energie" as opposed to enaji, like the English word "energy".