That actually has sparked a few questions of my own.

How does one gather the energy necessary to do magic and how does one increase their capacity for it?

Because as much as I can understand madness being a cost, the other cost is probably energy, right? Considering that "the more power one uses, the more energy one needs to sustain it" rule is in there.

So my only other question outside of the first is, what is the energy being mentioned here?