A method for optimized utilization of available energy resources/sources (3) of an electrical system such as for a vehicle generator system. The steps include: determining performance parameters LIst (e.g. current I, voltage U, temperature T, power PD (Pverlust), PE (Pwirk), efficiency, etc.) of a preferred energy resource (3); entering performance parameters LIst (current I, voltage U, temperature T, altitude of the vehicle, etc.) of the energy resource (3) into a thermal model (7); comparing a performance parameter LIst of the energy resource (3) with a specified performance limit parameter LSoll at a specified threshold level; determining a value for an energy reservoir ERest of a preferred energy resource (3); and to calculate a remaining time TMax depending on the energy reservoir ERest of a preferred energy resource (3) and using the one or more identified performance parameters LIst to maintain an operational state of the vehicle.