We have designed and implemented a tool to evaluate and optimize energy supply systems, so that the financial costs, the consumption of primary energy or the amount of harmful emission to the environment are minimized, We have set-up a general description of these systems and have applied a heuristic optimization method, Simulated Annealing, that allows us, in an efficient way, to determine the best way to fulfil different types of energy demand using a set of facilities of energy transformation and storage. We have also considered the time correlation introduced by the storage devices or transport processes and the possible behaviour of the system when the external conditions (fuel prices, demand profiles, etc.) change. Copyright (C) 2002 John Wiley Sons, Ltd.