1. A power control apparatus comprising: an inverter; and a power controller which (i) receives, as an input, a difference between a voltage reference value and an output voltage value output from a control target including the inverter, and (ii) outputs, to the control target, a control output for conforming the output voltage value to the voltage reference value, wherein the power controller includes: a matrix storage unit configured to store a coefficient matrix for representing, as a state space, a transfer function for the power controller determined by calculating a product of (i) a transfer function of an internal model obtainable by performing a Laplace transform on the voltage reference value and (ii) a transfer function of a partial controller, the transfer function of the partial controller being for outputting the control output after receiving, as an input, an output of the transfer function of the internal model; a state storage unit for storing a vector representing an internal state in the state space; and an operating unit configured to calculate the control output based on a difference between the voltage reference value and the output voltage value, the vector representing the internal state stored in the state storage unit, and the coefficient matrix, and wherein the transfer function of the partial controller is determined using an H∞control theory, so as to reduce (i) a first amount of control obtainable by multiplying the control output and a weighting function based on an amount of change in impedance of the control target and (ii) a second amount of control that is an output of the transfer function of the internal model.