MPG lost by adding speakers/amp

I would guess somewhere around 0.

 

there would have to be some loss even gm was saying that aftermarket systems can draw a fair chunk from the batteries.

 

Won't it depend on how much power your amp is going to draw?

 

And how heavy it is.
Several people have posted the expected extra consumption with headlights on, that will give you an idea of the added load on the electrical system for every 110 watts of draw from the system.

My guess, bugger all.

 

You are correct that there would technically be some loss because of the extra current draw but IMO it would be negligible in a real world situation.

 

Assuming we aren't talking about one of those insane competition systems. In that case the draw is like a welder.

Tom

 

2nd that.

 

The Prius minimum power output is about 40 HP, or about thirty thousand Watts. How big is your amp?

 

I once put a 6 channel system in a Suburban that used 3 SAE dual channel linear amplifiers rated at 1KW per side. The speakers could fold out of their door positions to provide sound for a whole field or party beach with the truck doors open. I needed to run the engine the whole time with a special alternator to power three separate DC-AC inverters for the system. I used an array of DC batteries as a sink to stabilize the draw on the alternator. This did have an effect on my fuel mileage, and my sobriety.