High-performance, timing accurate models of complex systems (called Virtual System Prototypes (VSP)) enable the computation of relatively accurate power in terms of events that occur in the model. The use of relative power computations by software, hardware and system architects enables them to optimize designs based on relevant objective functions, that include power, inter alia. The general form of a power computation function is given in the paper, as well as, an example of the implementation of a power calculator. The use of power, along with performance, as elements of an objective function, must drive algorithm choice and software development in mobility and other power-performance sensitive applications. The use of VSPs is mandatory in building optimal systems.