A mechanism is provided for estimating energy/power consumption of a fixed-frequency operating mode while system is running in dynamic power management mode. For each time interval in a plurality of time intervals within a time period: a first processor identifies a modeled total nominal power value for at least one second processor during a current time interval, stores the modeled total nominal power value for the current time interval in a storage, identifies a dynamic power management mode power value for the at least one second processor in the data processing system during the current interval, and stores the dynamic power management mode power value for the current time interval in the storage. Responsive to the time period expiring, a comparison is produced of a plurality of modeled total nominal power values and a plurality of dynamic power management mode power values over the time period.