  By employing the full counting statistics formalism, we characterize the
first moment of energy that is exchanged during a generally non-Markovian
evolution in non-driven continuous variables systems. In particular, we focus
on the evaluation of the energy flowing back from the environment into the open
quantum system. We apply these results to the quantum Brownian motion, where
these quantities are calculated both analytically, under the weak coupling
assumption, and numerically also in the strong coupling regime. Finally, we
characterize the non-Markovianity of the reduced dynamics through a recently
introduced witness based on the so-called Gaussian interferometric power and we
discuss its relationship with the energy backflow measure.
