The energy level of the little brother before and after his nap.
The two things that are being compared in the sentence are "my little brother" and "a ball of energy."
In the sentence, "Watching from my room, I saw that my little brother was a ball of energy after his nap," there is a comparison being made. Two things are being compared: the little brother and a ball of energy. The sentence uses a simile to describe the little brother by comparing him to a ball of energy.
