Energy Element

Energy Element (エネルギー元素), also called Energy Crystal (エネルギー結晶), is a type of energy resource from Mega Man 3 needed by Dr. Light and Dr. Wily to be used on the giant peace-keeping robot Gamma. They were scattered about uncharted planets, so the doctors sent robots to mine them. For some reason, these robots suddenly have gone crazy, and Mega Man set out with Rush to stop the robots and retrieve the Energy Elements protected by them. After fighting against Break Man, Dr. Wily steals Gamma and the Energy Elements. The Energy Elements were never shown in the game, and it's unknown what happened to them after Gamma's destruction, presumably being lost or destroyed.

Captain N: The Game Master
In episode 32 from the animated series Captain N: The Game Master, the Energy Elements are called Energy Tanks and resemble a large, yellow Energy Tank with a blue dome above. Mega Man obtains the last Energy Tank after defeating Top Man, and it is placed inside Gamma.

Mega Man Gigamix
The Energy Elements appear in the manga Mega Man Gigamix, and have an important role in the stories. In volume 1, they appear in the story Asteroid Blues, which is based on the events from Mega Man 3. In volumes 2 and 3 they are shown to have a connection with Duo and the Stardroids.

Mega Man (Archie Comics)
Before the Energy Elements had a direct appearance in the series, their presence was briefly implied in Mega Man Issue 20, when Mega Man briefly fought against Gamma when stuck in a time loop.

They also received a mention in Sonic the Hedgehog Issue 249, where Sonic the Hedgehog commented that it would take over a billion Energy Elements (when Mega Man explained that they were his world's most powerful energy source) to equate to the power output of one Chaos Emerald.

They first appeared in Mega Man #36, being shown as eight specially large and refined energy elements that Dr. Light connected in Gamma's mouth. These powerful crystal batteries are stolen by the Mega Man 3 Robot Masters in the same issue. They reappear in Issue 41, where it is revealed that Wily's third generation Robot Masters, per Wily's orders were going to let Mega Man have the Energy Elements anyways, although not without a huge fight. After all Energy Elements are recovered and installed on Gamma, Dr. Wily betrayls Dr. Light in issue 45 and steals Gamma.