The invention relates to an architecture of a propulsion system of a multi-engine helicopter, comprising turboshaft engines (1, 2) that are connected to a power transmission gearbox (3), and comprising a low DC voltage onboard network (7) for supplying helicopter equipment during flight, characterized in that it comprises: a hybrid turboshaft engine (1) that is capable of operating in at least one standby mode during a stable flight of the helicopter; an electrotechnical pack (20) for quickly restarting said hybrid turboshaft engine in order to bring said engine out of said standby mode and to reach a mode in which it provides mechanical power, said restart pack (20) being connected to said onboard network (7); and at least two sources (4, 16, 18) of electrical power for said onboard network (7).