In reply to Blue Straggler:
> (In reply to kipper12)
> [...]
>
> That's Top Gear for you
>
> Yes, the electrics will increase consumption, I always accepted that. I just didn't think they'd contribute SIGNIFICANTLY to the VARIATION that I am asking about.
I know that in an older car I had (mazda 121, original style), when sitting with the engine idling if I turned on the rear window heater the engine tone changed, so it was in some way a noticeable burden. How much, I don't know.
If you could get the wattage of the various systems (would be listed somewhere) you could work out total burden of various electrics in power. You could then look at the joule content of the fuel, run that through the efficiency of the engine (20%?) and the alternator (80%/90% ?) and work out how many litres per hour you'd use to keep those systems running. Then with your average journey speed you can calculate an effective mpg impact (faster you drive, quicker the journey, less important the electrics are).
Quick google indicated alternator might be giving somethign like a capacity of 650W, say 500W for ease of calc
http://www.physicsforums.com/showthread.php?t=267622
Say the alternator is 90% efficient, this means need shaft power of 556W into it. Say Engine is at least 20% thermodynamic efficient, then you need at most 2.8kW of fuel calorific energy in.
If you were driving for 1 hour, or 3600 seconds, you would then use about 10MJ. A litre of petrol has about 36MJ (
http://en.wikipedia.org/wiki/Energy_density ), so in one hour you'd use maybe 0.3l of petrol.
In UK gallons, that's 0.07 gallons.
In that hour, you might have driven 60 miles, maybe at 30mpg, so you'd have used 2 gallons of petrol.
So you end up that the electrics are maybe 3% of the consumption (assuming you're really using 500W of electrics... is that realistic/high/low?)
Then again, Futurama has a lesson to teach:
http://www.tvfanatic.com/quotes/this-ac-is-incredible-i-better-turn-on-the-...
(can't search youtube for the clip right now...)