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. <EOS>