5 amps is the highest current official USB cables will carry, and 48V is the highest voltage you can generally use as anything more is not considered intrinsica

5 amps is the highest current official USB cables will carry, and 48V is the highest voltage you can generally use as anything more is not considered intrinsically safe to handle -- it's the maximum "low voltage" Thus you get 5A @ 48V which is 240W.