The disclosure relates to a vehicle electrical system for a motor vehicle. The vehicle electrical system comprises: a low-voltage sub-system for at least one low-voltage load; a high-voltage sub-system for at least one high-voltage load; and a starter/generator; wherein the high-voltage sub-system is connected to the low-voltage sub-system by means of a coupling unit designed to draw energy from the high-voltage sub-system and to feed said energy to the low-voltage sub-system, wherein the high-voltage sub-system has a battery, which is designed to produce the high voltage and to output the high voltage to the high-voltage sub-system and which has at least two battery units having individual voltage taps, which are led to the coupling unit. The coupling unit is designed to selectively connect the battery units to the low-voltage sub-system. The disclosure further relates to a motor vehicle, comprising an internal combustion engine and such a vehicle electrical system.