You better go do some more research because that is totally wrong. The more load you put on the electrical system, the more the alternator has to work. Energy is coming from somewhere. The more load is on the alternator, the more engine power it takes to drive that alternator. The reason guys with large sound systems use deep cycle batteries is because they do just that.. they deep cycle... meaning the have a longer reserve capacity and are able to handle a load for long periods of time. A cranking battery cannot handle a long current draw on it for the same length of time that a deep cycle battery can. The alternator still has to charge the battery as it depleats. The alternator does not "make" power... it simply converts kinetic energy to electrical energy. The more is asked of it, the more it HP it takes to turn it.

Don't belive me? Start your car. Listen REAL close.
Turn all accessories off. Then turn on your headlights. You WILL hear the alternator load up and drag the engine down slightly.

Go put your vehicle on a dyno sometime. Run a dyno run with your headlights and all your electrical accessories running. Then make a run without. You WILL see a HP decrease when you run with accessories on.