[NeedMoreTea]

In the old days of lower power alternators (or really old days of generators), and lower power engines, the revs would drop noticeably, and the note change, when you turned dip lights on, aircon or even occasionally - in smaller cars - the heated rear demister! Made it very obvious the engine was working harder.

Now that tickover and fuel injection is computer controlled or we have EVs, and there's a far heavier electrical load in all cars all the time, you never notice any more. They all take power, and decrease fuel efficiency.