On-board power system for a vehicle, having - a first on-board power system branch having a first operating voltage U 1, the first on-board power system branch having a first energy store and a first electrical load, - a second on-board power system branch having a second operating voltage U 2, the second on-board power system branch having a second energy store, - a third on-board power system branch having a third operating voltage U 3, the third on-board power system branch having a third electrical load and an electrical machine, - a DC/DC converter designed to bidirectionally transmit energy at least between the first on-board power system branch and the second on-board power system branch, - a first switching apparatus, the first energy store and the second energy store being able to be electrically connected in series with one another via the first switching apparatus in such a manner that the third operating voltage U 3 can be provided by the first operating voltage U 1 and the second operating voltage U 2 together, characterized by - a fourth on-board power system branch having a fourth operating voltage U4, the fourth on-board power system branch having a fourth electrical load, a third energy store; - the first on-board power system branch also having a starter of an internal combustion engine of the vehicle, and the fourth on-board power system branch also having a generator; - a second switching apparatus, the first on-board power system branch and the fourth on-board power system branch being able to be electrically coupled via the second switching apparatus; - the third energy store and the second energy store being able to be electrically connected in series with one another via the first switching apparatus in such a manner that the third operating voltage U 3 can be provided by the second operating voltage U 2 and the fourth operating voltage U 4 together. - a first on-board power system branch having a first operating voltage U 1, the first on-board power system branch having a first energy store and a first electrical load, - a second on-board power system branch having a second operating voltage U 2, the second on-board power system branch having a second energy store, - a third on-board power system branch having a third operating voltage U 3, the third on-board power system branch having a third electrical load and an electrical machine, - a DC/DC converter designed to bidirectionally transmit energy at least between the first on-board power system branch and the second on-board power system branch, - a first switching apparatus, the first energy store and the second energy store being able to be electrically connected in series with one another via the first switching apparatus in such a manner that the third operating voltage U 3 can be provided by the first operating voltage U 1 and the second operating voltage U 2 together, - a fourth on-board power system branch having a fourth operating voltage U4, the fourth on-board power system branch having a fourth electrical load, a third energy store; - the first on-board power system branch also having a starter of an internal combustion engine of the vehicle, and the fourth on-board power system branch also having a generator; - a second switching apparatus, the first on-board power system branch and the fourth on-board power system branch being able to be electrically coupled via the second switching apparatus; - the third energy store and the second energy store being able to be electrically connected in series with one another via the first switching apparatus in such a manner that the third operating voltage U 3 can be provided by the second operating voltage U 2 and the fourth operating voltage U 4 together.