RPM drop and electric fans!

Just do the calculations:
Power (watts) = I (Current) x V (Voltage)
1 hp = 746 watts.

Let's just say your current is 40 amps, and your voltage drop is high, so you have 12.5 V, and your alternator is extremely inefficient, so its only 50% (should be closer to 70%)

40 amps * 12.5 V = 500 W.
500W/0.5 = 1000W (it's divide because its the load and not the generation)
1000W/746W = 1.34 hp.

That alone shouldn't drag the engine down much at all. If the rotors are trying to stop the rotation of the alternator, that would honestly be the only reason that would make sense but I find that to be somewhat unlikely.

I'm able to run an EFI ECU, Aeromotive 340 lph fuel pump, Contour fan, headlights, and wipers and keep the battery charged using a 78A Delco 12SI alternator. The stock style alternators have very low output at idle but that shouldn't mean that the engine drops in RPM so much. Of course my battery is still under the hood. It would pull the idle down somewhat but 50-100 rpm would be realistic and temporary until they are sped up.

Is the timing retarding when it drops below a certain RPM? Like vacuum advance hooked to manifold vacuum, springs are very light so they start opening even at normal idle speed, etc?