The ability for energy harvesting via the piezoelectric effect was studied for a unimorph element such as piezo buzzer. A simple equivalent circuit was proposed to predict the energy generated based on the internal stress. Unimorphs with a metal-cavity were used as a driving device of the generation panel. Both the AC open voltage and DC output voltage as a function of pressure period and number of element were measured. For the unimorph generation circuit, DC output voltage varies with pressure period, reaching a maximum value at $470{mu}F$. The maximum output voltage a according to load resistance was measured at $1M{Omega}$. Data analysis of the DC output voltage and time constant indicated that number of piezoelectric element of optimum was 60~80. It was found that piezoelectric unimorph has the possibility to be used as the driving element of the electric generation.