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 variable 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.