An ECU performs a controlling process including: switching a main converter to a boost converter CNV2 when a boost converter CNV1 is set as a main converter and when a first stress value Va is greater than a second stress value Vb; switching a sub converter to the boost converter CNV1; switching the main converter to the boost converter CNV1 when the boost converter CNV1 is not set as a main converter, and when the second stress value Vb is greater than the first stress value Va; and switching the sub converter to the boost converter CNV2.