  We present here a general method based on the investigation of the relative
energy of the system, that provides an unconditional error estimate for the
approximate solution of the barotropic Navier Stokes equations obtained by time
and space discretization. We use this methodology to derive an error estimate
for a specific DG/finite element scheme for which the convergence was proved in
[27]. This is an extended version of the paper submitted to IMAJNA.
