Clustered wireless sensor network models typically assume that a clear line-of-sight path separates the transmitter and receiver within a cluster; an accurate assumption??for relatively short distances. For longer distances, and in environments that are obstructed, the log-distance path loss model agrees better with measurements. In this paper, communication within a cluster is modeled using the free space propagation model for distances less than a crossover distance, d0, and the log – distance path loss model with loss exponent n > 2 for distances greater than d0. We determine the optimum number of clusters for minimizing the energy expended in the clustered WSN given the network geometry, the number of nodes in the network, the crossover distance d0, and the amount of data compression.