A method to determine the overall energy balance of neat PLA processing in a batch mixer was proposed. Energy consumption calculations rely upon shear rate and viscosity calculations using a double-Couette approximation. In the case of PLA mixing, it was found that the energy consumption is increased with an increasing rotor speed and decreased with an increasing temperature. During the melting phase the energy consumption is not strongly affected by the process parameters and the energy efficiency is close to 1. During the mixing phase, for a given viscosity, the energy efficiency is increased with increasing rotor speed or temperature and a method to optimise the process parameters to obtain a low viscosity with limited degradation and high energy efficiency was proposed. Moreover a comparison is given between the double-Couette approximation and conventional rheometry (rotational and capillary). Conventional rheometry translates into an overestimation of the viscosity especially at low temperature. This overestimation could probably be explained by the high sensitivity of rheology to molar mass reduction.