A controller has a predicting section which computes a predicted electric energy predicted to be supplied from a power distribution grid to the load during an assessment period which is a predetermined period. The controller has a supply electric energy computing section which computes a supply electric power based on the predicted electric energy, the supply electric energy being an electric energy to be supplied to the load from the electric power converter during the assessment period. The controller has a time computing section which computes a discharge time by dividing the supply electric energy by a rated power of the electric power converter. The controller has a control section which controls the electric power converter so that an electric power corresponding to the rated power is supplied to the load only for the discharge time within the assessment period.