A method for operating an energy coupling device electrically coupling a high-voltage on-board electrical system designed for d.c. voltage to a low-voltage on-board electrical system designed for d.c. voltage, having a high-voltage energy converter connected to the high-voltage on-board electrical system and a low-voltage energy converter connected to the low-voltage on-board electrical system, which are electrically coupled by means of a d.c. voltage intermediate circuit. The energy converters are controlled by a control unit. The degrees of conversion efficiency of the energy converters are ascertained and an intermediate circuit d.c. voltage of the d.c. voltage intermediate circuit is set as a function of a total degree of efficiency produced by the sum of the degrees of conversion efficiency in order to maximize the total degree of efficiency.