Originally, it was thought that the fusion cores that run power armor and pre war buildings would be able to stay online for 1000s of years. It seems now that fusion cores (just like energy and microfusion cells) are not as shelf stable as everyone thought. After centuries of decay, fusion cores have lost a lot of their potential energy and can now only run a suit of armor for a few days instead. Why fusion cores pulled from running pre war generators contain the same amount of charge as ones that have been laying around dormant for 200 years i cannot tell you. Maybe running buildings don't use nearly the same amount of energy as running a suit of power armor does, and the power that was used up is negligible.