Elemental Energy is a mysterious force used by many beings in the Matoran Universe to control their element-based abilities. When this energy reserve is used up, a being can no longer make use of their powers until it is restored or recharged.
Elemental energy should not be confused with Toa Power, which can never be replenished. Upon becoming a Toa, they form a permanent affinity to an element, and thus gain vast amounts of natural elemental energy. However, while Toa energy cannot be used up from the usage of their powers, it can be forcibly drained with the use of a Nui Stone.
Once a Toa has fulfilled their destiny, they may choose to become a Turaga, which immensely reduces their elemental energy stores, and expends all of their Toa energy, with it either dissipating into nothingness, or being used to create Toa Stones, in order to bring forward a new generation of Toa.
Elemental energy can be restored by resting, or it can be recharged quickly by utilizing the Mask of Elemental Energy.