Your situation is very common. Turn your brightness down and you will save a load of power. However if the os is sucking 46% of energy used then something is very wrong. I'd be worried if it strayed above 10% so you might have a faulty device.
Of course all of this depends on what you want to do with the device after root. There is no point if you want to stick custom ROMs on it as being a nexus device it will constantly be updated. Only if you require root access to a specific applicat...