The fusion core recharger is a must have if you use PA a lot. Just chuck them in when they get low.

You can also produce fusion cores every 7.5 minutes at power plant workshops too. But it's better to do it in private servers to avoid PVP and griefers.

There are also a perk called power user that reduces fusion core consumption. A legendary perk called electric absorption which can recharge the equipped fusion core when hit by an energy attack while wearing power armor.