electrical load on your alternator is mechanical load on your engine.  1 hp = 746Watts. your headlights and other lights are about 130 watts, your blower fan about 120 Watts.  It translates to about 1/3 to 1/2 Hp to drive it all, plus some mechanical losses on bearings, etc, but usually count alternators as 95%efficient. Idle should not get affected this much, unless your engine is running lean.  Try changing the 02 sensor, might give a slightly different reading then it should.  FIRST, Check to see if your Idle circuit works, Maybe your idle air control is not fuinctioning at all.