Methods and apparatuses for transient analyses of a circuit using a hierarchical approach. In one embodiment, the cells are grouped locally on the power supply network according to average power dissipation. A time varying current of each cell group is estimated using a probabilistic approach to represent the cell group so that the probability of a more severe waveform for the current of the cell group is under a certain level. For example, the cells in a group are partitioned as switching cells and non-switching cells using cell toggle rates for the determination of the time varying current. The circuit model of the power supply network includes the current sources according to the estimated time varying currents for the cell groups, the power supply wire resistance, the power supply to ground wire capacitance, well capacitance and the de-coupling capacitance from non-switching cells.