Wireless sensor network consisting of a large number of small sensors with low-power can be an effective tool for the collection and integration of data in a variety of environments. Here data exchange between the sensors and base station need to be designed to conserve the limited energy resources of the sensors. Grouping the sensors into clusters has been recognized as an efficient approach for saving the energy of the sensor nodes. In this paper we propose an analytical model based on homogenous spatial Poisson process, which allows the number of clusters in a sensor network minimizing the total energy spent for data exchange. Computer simulation on various size sensor networks with the LEACH algorithm reveals that the proposed model is very accurate. We also compare the proposed model with an earlier one, and it turns out that the proposed model is more accurate.