Power estimation is one of the key aspects that can help designers create digital circuits more effectively. If a designer is able to estimate circuit parameters during the early stages of development, correct decisions can be made that can significantly shorten the design time. The early design stages are represented by modeling at the system level of abstraction. However, existing system-level power/energy estimation methods are either too complicated, or they do not consider power management when estimating power consumption, meaning they are inaccurate. Therefore, in this paper we propose a method for a more accurate system-level estimation of the dynamic energy consumption by considering the impact of power management. The SystemC description of a power-managed system and the simulation results (in the form of the value change dump (VCD)) are inputs to the estimation method. The proposed method is based on an activity profile using the modified Hamming distance computation. The method is especially useful for the exploration of alternative power-management strategies, and it helps the designer to select the most efficient strategy.