A power conservation system in a computer system which includes a processing unit operating under control of an operating system. The computer system generates distinct call functions to the operating system where each call function is either in an active class or an idle class. The power conservation system has a plurality of states of operation including an ON state, a DOZE state, a SLEEP state and an OFF state. An activity monitor monitors the activity of the computer system and generating control signals for selecting one of the state of operation for the computer system. The activity monitor includes a storage for storing a call value for each distinct call function and activity threshold values for the various states of operation. The call values are weighted for the call functions whereby different call functions have a greater or lesser impact on the value of an activity level. The call value for each call function is retrieved when the call function is made to the operating system. The retrieved call values are sequentially accumulated to form an activity value which indicates the activity level of the computer system. A comparator compares the activity value with the threshold values and in response to the comparison generates a control signal to a power controller which selects the states of operation for the computer system, thereby regulating the power consumption of the computer system based on the activity of the computer system.