  In this paper, we introduce a real symmetric and positive semi-definite
matrix, which we call the non-equilibrium conductance matrix, and which
generalizes the Onsager response matrix for a system in a non-equilibrium
stationary state. We then express the thermodynamic efficiency in terms of the
coefficients of this matrix using a parametrization similar to the one used
near equilibrium. This framework, then valid arbitrarily far from equilibrium
allows to set bounds on the thermodynamic efficiency by a universal function
depending only on the degree of coupling between input and output currents. It
also leads to new general power-efficiency trade-offs valid for macroscopic
machines that are compared to trade-offs previously obtained from uncertainty
relations. We illustrate our results on an unicycle heat to heat converter and
on a discrete model of molecular motor.
