1. A power controller designing method for designing a power controller which receives, as an input, a difference between a voltage reference value and an output voltage value output from a control target including an inverter, and outputs, to the control target, a control output for conforming the output voltage value to the voltage reference value, the power controller designing method comprising: setting a weighting function based on an amount of change in impedance of the control target; and determining, for the power controller, a transfer function composed of a transfer function of an internal model obtainable by performing Laplace transform on the voltage reference value and 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, wherein the determining includes determining the transfer function of the partial controller using an H∞control theory and determining, for the power controller, the transfer function of the partial controller by calculating a product of the transfer function of the partial controller and the transfer function of the internal model, so as to reduce (i) a first amount of control obtainable by multiplying the control output and the weighting function and (ii) a second amount of control that is an output of the transfer function of the internal model.