Energy

From Liquipedia StarCraft Brood War Wiki

Energy is a regenerative resource on certain units in StarCraft. It is used for casting spells and abilities. The only auto-casting ability in the game that requires energy in the Medic's Heal.

All energy-using units start with a maximum of 200 energy, which can be upgraded to 250. When units with energy spawn they start with 25% of their maximum energy (50 or 62 energy respectively), the exception being the Shield Battery which starts with 100 of its maximum 200 energy. Note that units that begin construction (the progress bar for them has started) before their energy upgrade is finished will not receive the bonus 12.5 energy. They will only receive the bonus energy if they have started construction after the energy upgrade has finished[1]