do you know the equation for efficiency? if not, do you think you can derive it? if a system were 0 percent efficient(0.00), you'd need to supply infinite energy to reach any finite stored energy. If the system were 100% efficient (1.00), you'd need to supply Z joules to store Z joules. Can you think of a relationship between P_in and P_stored that relies on the 0 to 1.00 representation of efficiency that would produce the end behavior we just explored?