the point is: what do you want to compare? of course the two graphs you obtain should have a similar shape, but it could be that you will need some arrangement if you need to compare the values.
I don't know what your friend meant with "energy", but he is absolutely right when he speaks about normalisation. Let's imagine you want to compare the sensisivity of the two accelerometer: you need to have the data belonging to the same interval. the output interval depends on several things like accelerometer itself, ADC resolution, maybe scale settings and so on. in that case, you should normalise on "output dynamic" or "output range".
If your problem is more general, let say is "what is the normalisation?", imagine the situation I described above, and suppose you have the data from the first accelerometer coming from a 10bit ADC, and from the second one coming from a 12 bit ADC.
The first bunch of data will belong to the [0,1023] inverval, the second one to the [0, 4095] interval. In this case, if we suppose that both accelerometers have the same sensitivity and max acceleration range, when the body is moved at 75% of his maximum acceleration the first one will give you 75% of 1023, the second one 75% of 4095...these two data represent the same acceleration but are clearly different.
To compare mathematically the two bunches of data, in this case, you should just make the ranges equal with some operation that is called "normalisation". In this case, you could simply use the range of the first accelerometer as "normalised range" and divide all the data coming from the second one by 4, or use float numbers and divide the first by 1023, the second by 4095 normalising to the normalised range [0, 1].
hope this helps