Models that can be used to analyse the fuel saving potential of electrically driven auxiliaries in heavy vehicles are presented. With the purpose of evaluating the influence on fuel consumption from various concepts and control principals, a model library is developed in the modelling language Modelica. The library contains a mixture of models developed from physical principles and models fitted to collected data. Modelling of the cooling system is described in some detail. Simulation results are compared with measurement data from tests in a wind tunnel.