1. A computer program product encoding a computer program for executing on a computer system a computer process for determining an amount of a resource consumed during a simulation interval in a performance prediction simulation of a system including software and hardware elements, the computer process comprising: calculating an aggregate resource requirement associated with the resource in a simulation interval; calculating a system event horizon for the simulation interval based on a resource usage state change in the system; and adjusting the aggregate resource requirement based on the system event horizon to provide an adjusted aggregate resource requirement for the resource, wherein the adjusted aggregate resource requirement represents the amount of the resource consumed during the simulation interval.