Simply put- accessories driven off the engine all produce parasitic loses in power of some kind, at some time.
As an aside, but related, before 1971, engines were rated in gross horsepower- the engine itself, not even installed in the vehicle, no alternator, a/c compressor, nothing. That's why you'll see old ads for the "450 hp" Chevelle SS LS6. That was 450
gross hp. The LS3 spanks it in terms of hp. The V6 Camaro makes more brake hp than a lot of V8s of the golden era of muscle.