If you are walking instead of running when using power armor the cores last forever.
At level 29 I finally started using my power armor because Nick found himself a frame and hasn't been out of it since. In that time I had collected enough pieces to upgrade both sets. But man is it a pain to have to repair them both because I have to take the pieces from nick, step out of my power armor, remove my core, then repair everything, put my core back in and stop into my power armor and then give the pieces back to Nick and tell him to re-equip them. I wish we could just walk up to the power armor station and repair the pieces without taking the frame off.
As a side note, I've noticed that when I am in power armor I can hit things and when I don't have power armor on I am a horrible shot even if the weapon has 100% accuracy rating. Is it because I'm shooting from a "sneak" position that my aim is so horrid or is this the difference between shooting in power armor and not?