  The excess work required to drive a stochastic system out of thermodynamic
equilibrium through a time-dependent external perturbation is directly related
to the amount of entropy produced during the driving process, allowing excess
work and entropy production to be used interchangeably to quantify dissipation.
Given the common intuition of biological molecular machines as internally
communicating work between components, it is tempting to extend this
correspondence to the driving of one component of an autonomous system by
another; however, no such relation between the internal excess work and entropy
production exists. Here we introduce the `transduced additional free energy
rate' between strongly coupled subsystems of an autonomous system, that is
analogous to the excess power in systems driven by an external control
parameter that receives no feedback from the system. We prove that this is a
relevant measure of dissipation -- in that it equals the steady-state entropy
production rate due to the downstream subsystem -- and demonstrate its
advantages with a simple model system.
