Energy saving is a very important design consideration in wireless sensor networks. In cluster based wireless sensor networks, the energy could be saved when the nodes in a cluster that have data to transmit participate in cooperative Multiple Input Multiple Output (MIMO). In this paper, by making the idle nodes, that have no data to transmit, participate in the cooperative MIMO it is found that much more energy could be saved. The number of the idle nodes that participate in the cooperative MIMO can be optimized to minimize the total energy consumption. It is also found that the optimal number of all the nodes participating in cooperative communication does not vary with the number of nodes that have data to transmit in the cluster. The proposition is proved mathematically. The influence of long-haul distance and modulation constellation size on the total energy consumption is investigated in this paper as well.