The eco power sphere is a Precursor artifact in Jak 3. It has four slots, each one fitting one light and one dark eco crystal, meaning that four of each are needed to power the sphere, which powers the Planetary Defense System. The eco power sphere powers up in a matter of minutes.
It is unclear where Jak first acquired the sphere, as he had it upon entering the catacombs. It is possible that it was on the defense platform and Jak simply inserted the crystals in the sphere. As Seem said, the forces of light and dark form great energies, it would seem that when eight are brought together, they form incredible power, subsequently allowing the Precursors to destroy the Dark Maker ship in a single shot.