An on-board power system for a vehicle includes a first on-board power system branch with a first operating voltage U1, a first energy accumulator, and a first electrical consumer; a second on-board power system branch with a second operating voltage U2 and a second energy accumulator; and a third on-board power system branch with a third operating voltage U3 and a third electrical consumer. The on-board power system also includes a DC/DC converter configured to transmit energy bidirectionally at least between the first on-board power system branch and the second on-board power system branch. The on-board power system also includes a switching device configured to selectively connect the first energy accumulator and the second energy accumulator to one another in series via in such a way that the third operating voltage U3 can be made available by the first operating voltage U1 and the second operating voltage U2 together.