This is a difficult question. Usually, the energy consumption is directly related to the CPU load and I/O load (especially hard disk accesses). It is difficult to simulated all this in Peersim, but you can try to evaluate the participation of each node your simulating in order to have a simple model of the load of each node, and therefore be able to make an estimation of the global load.
Its obviously very difficult to have an estimation of the energy consumption in a real world, as it is hardly depends on the type of host that are particiatng in the network (CPU, disks, etc...).