A portable computer system with a battery and a processor, a memory, a display and storage device; all powered by the battery which includes a power management circuit configured to control the operation of the processor and memory in order to vary an amount of power consumed. The power management circuit is configured to accept a requested operating time and is configured to signal the power management circuit to modify the operation of the processor, memory, storage device and display so that the portable computer system can operate from the battery for said requested operating time.