User blog comment:JmLyan/Jace, the Mind Sculptor/@comment-3974211-20130408163140/@comment-3974211-20130408185752

I still think energy would be more appropriate, even if you rename it to "Mana Reserve" or something fancy that suits his theme. Now that he cannot buy mana, he has to buy mana regeneration (which is slow even if you have lots of it). He will literally run out of mana and it will never regeneration. Using energy means he has short bursts and rapid recovery - it prevents him from entirely mitigating his allies costs, while never truly being able to "run out of mana" (which is an issue of mana champions).