Energy Stores are vital if you want your THEL to maintain fire during a prolonged battle - Energy Stores are more than just a battery for energy. After completing the last two missions on hard, i'm pretty sure about this, but more Energy Stores either improve the efficiency of THELs, or get energy feedback, or generate their own energy, i don't know, but 21 maxed solar panels couldn't maintian just one THEL, whereas less than half that many with say 3 energy stores and it's a different story ;)
 "The Space Game: Missions"){kind=icon}
 "Infectonator : World Dominator"){kind=icon}