Since you are a classical based guitarist, i will assume you know a bit about music theory:


Basically it is the tonic and the 5th plus the supertonic

For example a C power chord (C5th) will be C G C


or look like this

E-------------
B-------------
G---5---------
D---5---------
A---3---------
E--------------

They are moveable.

E---------
B---------
G---------
D---3-----
A---3-----
E---1-----

That is the F power chord. Those two shapes are generally used for the power chord shape.