As the game progresses you will be able to get your hands on Fusion Cores more readily but at the start, use sparingly. Power Armor needs to be powered by a Fusion Core and these do deplete so you won’t be walking around in Power Armor for the whole game. Therefore it might be worth holding off on upgrading until you have the required perks to save on materials. It’s worth noting that the upgrade materials required are the same from say Model A to B or A straight to D.
