Recent studies have demonstrated that it is feasible to harvest energy from raindrop. A challenge in designing a raindrop energy harvester is the rain droplet would accumulate on the surface of the harvester and affect its performance. In a previous work, we have modelled the dynamics of a piezoelectric beam subjected to water droplet impacts with a water layer formed on the surface. This work presents a theoretical model to describe the transient dynamics during the formation of water layer on the beam. The average water droplet impact force is described by a partially inelastic impact coefficient that varies during the formation of water layer. The maximum root mean square voltage measured experimentally is 0.05 V with an average percentage error of 6.94% compared to the theoretical model. Experimental result revealed that the optimal performance of the harvester occurs before the water layer spreads to the width end of the beam.