A histogram difference method and system for power/performance measurement and management has low data storage requirements while supporting multiple monitoring applications having different update rates. Histogram data for power usage and/or performance mode is collected at a predetermined rate and the histogram data is read out at periodic intervals by the monitoring applications. The monitoring applications subtract the histogram data from previously read histogram data set to determine a interval difference histogram. The minimum and maximum values for the interval are the lowest-valued and highest-valued bin in the interval difference histogram that have a count greater than zero. The average value for the interval is the mean of the interval difference histogram. A conservative bound of the maximum and minimum values for a system can be determined by adding the values of the maximum and minimum values determined for each subsystem in the system.