The presentation describes a design procedure based on multi-objective genetic algorithms and a backward simulation tool named PLA.N.E.S. The input parameters were the size of engine and electric motor, the specification of the battery and the battery management strategy. The procedure was applied to an unmanned aerial vehicle. The goals of the optimisation were minimisation of fuel consumption, maximisation of electric endurance, minimisation of the powertrain volume and maximisation of the performance at takeoff.