Punching power is a composite of speed, technique, accuracy, heavy-handedness, explosiveness and intent. You can add in stamina and boxing ability as well when applied to actual KO ability in the ring. Of the above, speed, heavy-handedness and explosiveness are physical traits. You can train them up, but by and large you have them or you don't. Stamina is also a physical trait, but one of the easiest to improve by a significant margin. Without stamina punching power rapidly drops off. Technique, accuracy, and boxing ability (ability to deliver hard, powerful punches on a moving, aggressive target) are things that are learnt and honed through hours in the gym. Intent is a tricky one to categorise. I'd say it's primarily in-born, and forms your basic fighting psyche. Without real intent to harm your opponent your punches will lack that savageness that the best punchers have (Tyson, Dempsey etc), but it's something that can fluctuate depending on external circumstances. A fighter who threatens one's family or insults one's race or religion etc can light a fire up anyone's ass.