Grid based wireless sensor networks have the advantages of dynamic topology configuration and flexible selection of forwarding paths. However, like other schemes, the energy management of electronic device is a very important issue when the lifespan of the network is critical. The energy consumption of a grid based network depends on the topology of the grid as well as the actual forwarding path. In this paper, we analyze the upper and lower bounds of the transmission energy costs for grid based wireless sensor networks. The results are applicable for evaluation of the effectiveness of grid construction scheme and the routing efficiency of grid based networks. In order to illustrate the applications of the derived results, we proposed an example of topology combination approach to evaluate efficiency with respect to the derived lower bound. Experiments were conducted to verify the validity of the derived bounds and evaluate the energy consumption of the multiple sources forwarding through topology combination.