  In this paper, we derive a number of inequalities which express
power-efficiency trade-offs that hold generally for thermodynamic machines
operating in non-equilibrium stationary states. One of these inequalities
concerns the output power, which is bounded by a quadratic function of the
thermodynamic efficiency multiplied by a factor. Different factors can be
obtained according to the level of knowledge one has about the underlying
dynamics of the machine, they can depend for instance on the covariance of the
input flux, the dynamical activity, or the non-equilibrium conductance.
