In this paper, a general model of multibit Differential Power Analysis (DPA) attacks to precharged buses is discussed, with emphasis on symmetric-key cryptographic algorithms. Analysis provides a deeper insight into the dependence of the DPA effectiveness (i.e., the vulnerability of cryptographic chips) on the parameters that define the attack, the algorithm, and the processor architecture in which the latter is implemented. To this aim, the main parameters that are of interest in practical DPA attacks are analytically derived under appropriate approximations, and a novel figure of merit to measure the DPA effectiveness of multibit attacks is proposed. This figure of merit allows for identifying conditions that maximize the effectiveness of DPA attacks, i.e., conditions under which a cryptographic chip should be tested to assess its robustness. Several interesting properties of DPA attacks are derived, and suggestions to design algorithms and circuits with higher robustness against DPA are given. The proposed model is validated in the case of DES and AES algorithms with both simulations on an MIPS32 architecture and measurements on an FPGA-based implementation of AES. The model accuracy is shown to be adequate, as the resulting error is always lower than 10 percent and typically of a few percentage points.