Can you explain this comment please? I understand the benefit of the efan over the stocker, but unlike the A/C clutch/compressor, the alternator is ALWAYS running and I've never seen where an electrical load on it directly relates to an appreciable and measurable loss in HP/torque...
This is pretty much spot on:
I believe that the magnetic force between the rotor and stator increases within the alternator, depending on the current draw. More power ie large electric fan = more force to spin the pulley.
Most people fail to realize(or simply refuse to realize) that there is a correlation between engine load and electrical load, and that electricity is not free. If electricity were free, then we would just strap a powerful electric motor under the hood and have a free power source. But sadly this isn't the case.
Basically, an alternator spins freely when not energized(not under any load). However, when an alternator is energized(has a load on it) it acts like an electromagnet and there is resistance. The amount of resistance depends on the load. That's pretty much it as far as how it works.
You can relate this to hp...keep in mind I am not an electrical engineer, but am nearly positive this is correct:
Say you have a 150amp alternator at full load
A typical alternator puts out 14.4volts (roughly).
150amp * 14.4volts = 2160watts.
2160watts / 745.7 = 2.9hp.
Figure anywhere from 10%-50% efficiency, lets say 25%): 2.9hp * 1.25 = 3.6hp
So basically, you use 3.6hp. Keeping in mind this is at FULL LOAD. Your alternator is nearly never at full load. A stereo head unit may use ~20amps, an efan may use ~30amps, headlights, etc, etc. So this is the reason you see vehicles today with higher amp alternators(more electronics to run).
Hope this helps...