Energy is the resource that is translated into a damaging laser by the player's ship. The energy capacity and regeneration differ from from ship to ship and can be upgraded linearly in any ship other than tier 7s.
The level of energy can be gauged via the yellow bar found on the top left of the screen when in game. The bar will appear darker when the main pulse can't be shot due to either the cannon's reload time or the lack of energy. When the main pulse becomes available again, the game will play a sound indicating to the player that it can be shot and the energy bar turns bright yellow again.