One thing about the Power Amour in the game Fallout 4 is that you will need to constantly search for fusion cores so that the Power Amour is always active. Here is a list of places where you can collect these Fusion Cores.
After completing the third quest, you will need to get to Nick Valentine’s detective agency in Diamond City. There, you will be handed the key to Kellogg’s house. At the ground floor, crouch to press the red button at the desk and you should have access to four of these Fusion Core.
Another place to find them is at Fort Hagen. Explore the side to find a room at the western edge of Commonwealth. Head to the generator room and get the fusion core from the engine. You can also head to The Castle to obtain the fusion core although you will need to eliminate the aquatic opponents first.
There are also Fusion cores to be found at the Weston Water Treatment Plant. These are just a few of the places you can find the Fusion Core. Happy searching.