  The work content of non-equilibrium systems in relation to a heat bath is
often analyzed in terms of expectation values of an underlying random work
variable. However, we show that when optimizing the expectation value of the
extracted work, the resulting extraction process is subject to intrinsic
fluctuations, uniquely determined by the Hamiltonian and the initial
distribution of the system. These fluctuations can be of the same order as the
expected work content per se, in which case the extracted energy is
unpredictable, thus intuitively more heat-like than work-like. This raises the
question of the `truly' work-like energy that can extracted. Here we consider
an alternative that corresponds to an essentially fluctuation-free extraction.
We show that this quantity can be expressed in terms of a non-equilibrium
generalization of the free energy, or equivalently in terms of a one-shot
relative entropy measure introduced in information theory.
