I want to get the information about the energy in the node, so those neighbouring nodes can reroute the data packets when the neighbouring node energy is less.
Currently UnetStack simulator doesn't provide energy measurements directly. However, it's not hard to do yourself for simulations. See this discussion for some suggestions:
The current version of UnetStack does not have any energy model per se. But the trace and logs provide you all the information you'll need (transmit/receive counts, simulation time) to compute the energy consumption. Specifically, you'd want to assign some energy per packet transmission, some energy per packet reception, and some power consumption while idling. If you dynamically adjust power level or packet duration in your protocol, you will need to account for that too.
Practical devices that use UnetStack often have a battery voltage parameter that provides some measure of energy available. However, this may be hard to use as battery voltage does not linearly depend on energy, but is highly dependent on the actual battery chemistry.
Something else that you might want to bear in mind in developing routing protocols that use energy information: transmitting remaining energy information from a node to neighbors takes energy! Do keep this in mind!!!