If you're playing Fallout 4 and looking to power your Power Armor, then you'll need to know where to get Fusion Core. These can be found scattered around the world, won by defeating enemies or sourced through quests.
One of the most important things to understand about Fusion Cores is that they are like uranium in that once they're depleted, you can't recharge them with any other resource. This means that when you run out of them, you'll have to find new ones somewhere else in the Commonwealth, or buy some from vendors.
There are a few places where you can find Fusion Cores in the game, but they are not easy to find, and they usually take a while before you'll have enough. This is why it's a good idea to make sure you have a supply of them on hand at all times, so you can always keep your Power Armor running.
Fort Hagen: Deep underground in the basement of this fort, you can find a fusion core that will help you get started with the main storyline. The only problem is that you'll need to kill two Synths before you can get access to the key and the room where the core is located.
Red Rocket Truck Stop: North west of Concord, near the start of the game, this truck stop is one of the first locations that you'll come across. This place is a little tricky to get to, so you'll need to use bobby pins to hack the door.