Actually, this sounds similar to the Spell Point system in Iron Heroes, and the "Unlimited Mana" system for GURPS (which already uses fatigue points to power spells). In both, every arcane caster has a budget of "mana points" he can use safely per day. The caster can spend *over* that budget, but the further in debt he goes, the more likely spells will fail, backfire, or take effect on the wrong target. Mana points return on an hourly or daily basis, but (at least with Umana) at a fraction of the mage's total mana capacity.
Originally Posted by upidstay
With "Unlimited Mana", mages can reliably cast incredibly powerful spells ... once or twice a day. After that, they have to recover their mana, or become a walking time bomb.
"On two occasions I have been asked [by members of Parliament], 'Pray, Mr. Babbage, if you put into the machine wrong figures, will the right answers come out?' I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question."
- Charles Babbage (1791 - 1871)