On-board electrical system of a motor vehicle and vehicle with such an on-board electrical system

The invention relates to an on-board electrical system (1) of a motor vehicle and a method for operating an on-board electrical system for a motor vehicle, in particular for a commercial vehicle. The on-board electrical system (1) comprises a first sub-network (2) in which a first nominal voltage (U1) is applied, a second sub-network (3) in which a second nominal voltage (U2) is applied, and a voltage converter (4) which bidirectionally connects the first sub-network (2) to the second sub-network (3). The first sub-network (2) comprises a first energy storage device (5) and a first load resistance (6) formed by multiple consumers. The second sub-network (3) comprises an electrical machine (10) which is designed for starting an internal combustion engine of the motor vehicle and for generator operation or recuperation operation, and a second energy storage device (9) which is connected to the electrical machine (10) and is designed to store electrical charge generated by the electrical machine (10) during generator operation or recuperation operation. The voltage converter (4) is designed to transfer energy between the first (2) and second (3) sub-network via the voltage converter (12) as a function of a state of charge of the first energy storage device (5) and/or a state of charge of the second energy storage device (9).