Energy Crystal

Energy Crystal, known as Energen Crystal (エネルゲン水晶) in Japan, E-Crystal (Eクリスタル) and EC for short, is an energy resource from the Mega Man series used by Reploids and other machines.

Mega Man X series
Energy Crystal is mentioned in some games from the series. In Mega Man X2, Crystal Snail took over an Energen Crystal mine. Tunnel Rhino from Mega Man X3 originally worked in an Energen Crystal mine before becoming a Maverick. In Mega Man X: Command Mission, the Gimialla Mine was an Energen mine.

Mega Man Zero series
In the Mega Man Zero series Energy Crystals appear as items that serve as currency and are also used to grow Cyber-elves by feeding E-Crystals to them. In Mega Man Zero 3, the Mod Cards 19 and 52  change the appearance of the E-Crystals.

As there is an energy crisis during the series, Dr. Ciel created the Ciel System (System C.I.E.L. in Japan, Convergent Ignition by Energen Linkage) between Mega Man Zero 2 and 3 to solve the problem, which uses Energy Crystals as a catalyst. The Resistance Base is also dependent upon E-Crystals, especially the teleporter in the commander's room as it uses plenty of power. In Mega Man Zero 3, a Giant Elevator was used to bring up a huge cache of Energy Crystals under Dr. Weil's orders. The exact reason Weil needed the energy is never revealed, although it most likely has something to do with Weil controlling the Reploids around the world via Omega and the Dark Elf. The Ragnarok from Mega Man Zero 4 contains a superconductive Energen fusion core and a giant Energen fusion plasma beam.

Mega Man ZX series
Like the Mega Man Zero series, Energy Crystals appear as items that serve as currency. In the first Mega Man ZX game E-Crystals are also used to repair damaged Biometals.

Rockman X manga
In the Rockman X2 manga, Crystal Snail took over an Energen Crystal mine.

Rockman X The Novel: Irregulars Report
In the novel, Energen Crystals are an energy source discovered by Dr. Light late in his years that he used for Mega Man X, X's capsule, and many of the machines in his laboratory, which was built atop a vein of Energen Crystals and allowed the machines to continue working for several years after he passed away. They had no name when Dr. Cain learned about them from Dr. Light's notes, using the placeholder name "Energen Crystal" that was in the notes as a homage for Light's findings and announcing their existance to the world three years after he found X. This earned Cain the position as well as financial power of being the maker of an energy of a new age, and he used that wealth to devote himself to the research of X, the progenitor of Reploids, who would also use the crystals as their primary energy source. While going to a Energen mine, X considered using raw crystals to replenish his energy despite their their purity not being optimal for that.

Rockman Zero manga
When Zero gets hungry, Lito tries to give him human food, but Zero says he is unable to eat it and Lito wonders what a Reploid eats. Later, a Reploid from the Resistance gives an Energy Crystal for Zero.

Several years later, Omega needs many Energy Crystals to power the Giant Omega-DX, and steals several from Lito's house. The new Zero saw Omega and the two fight. Omega manages to escape, but without noticing his sack was damaged in the battle and he left a trail of Energy Crystals. Zero retrieves all Energy Crystals and finds Omega, fighting against him and his minions. When his minions are destroyed, Omega starts crying as he will be unable to build the Giant Omega-DX alone in three days and without Zero's Energy Crystals, saying Dr. Weil will fire him. Zero takes pity of him and gives the Energy Crystals to Omega, saying that he will help him in the construction.

Trivia

 * The Energy Tank was misnamed "energy crystal" in the North American Mega Man 2 instruction manual.