The idea of supplying energy for low-power devices such as MEMS and remote sensors by converting the ambient vibrations to electrical energy through energy harvesters has been subject to many papers in recent years. One typical energy harvester is a cantilevered beam with one or more layers of piezoelectric material (PZT) connected to a harvesting circuit by means of electrodes. The literature includes attempts to provide the electromechanical model of this type of energy harvesters and one frequently used approach is the single-degree-of-freedom (SDOF) modeling which is only applicable for the case of excitation around the resonance frequency. In this work, the exact analytical solution of the base excitation problem is given for a clamped-free Euler-Bernoulli beam since the excitation of the harvester is generally due to its base motion in real life applications. The base motion is described by translation and small rotation and is not restricted to be harmonic in time. The general solution is then reduced to the case of harmonic base excitation for comparison with the corresponding SDOF model. It is shown that the commonly accepted form of the SDOF model may yield highly inaccurate results. A correction factor, which should also appear in the electromechanically coupled equations, is introduced for improving the SDOF model.