I do want to mention I did not say the AC is electrically driven!
Even if I rated it in Watts, the strain on an engine from a 300W radio is about the same as an AC.
The mechanical pull or drag on an engine can be calculated and converted into Watts!
Another thing is that the pull on the alternator, at low or high rpms is always the same for voltage regulated applications like cellphone chargers, dvd systems, or small LED TV's. A fan for instance, draws slightly more if the alternator rotates faster, due to alternator 'charging' the battery with a minimal voltage increase at higher rpms. So the fan would receive a higher voltage too, causing it to be able to rotate faster, moving more air, and thus drawing slightly more power.
A few other things to consider, when driving at low RPM's (500-1200rpm) or idling, on a non-diesel engine, most engines are running less efficient. In other words, if you're driving in a gear at 1100rpm, charging your GPS system might waste more gasoline (even though it uses the same power draw at higher engine rpms); but it are fractions of a tenth of a mile per gallon; you won't be able to record it.
And the fact that driving in higher gears automatically cost you more fuel nullifies the thought whether it's better to charge systems at engine idling or while driving.
But what would affect your mileage much more would be driving in lower gears (1-3 or 1-4) or driving in the city.
Suppose you have 2 parallel roads that go straight from your house to your destination, where both roads have no red lights (or stop signs all the way).
On one road you can drive only 35mph, on the other 50+mph.
You would be wasting more gas driving the 35mph road in 3rd or 4th gear, than driving the 50+mph road in 4rth or 5th gear, with all your electronics charging,and fans on.
No: electrical draw from the alternator won't effect engine load or make a difference in your gas mileage.
As others have stated though, the air conditioning will.
This is because the turning on the AC engages the clutch requiring the engine to turn an additional load (the AC compressor). Using electricity, however, does not add any load to the already-turning alternator. The alternator is engaged & loading the engine continuously and is never easier or harder for the engine to turn: its resistance does not fluctuate.
Prodigit: "Another thing is that the pull on the alternator, at low or high rpms is always the same for voltage regulated applications like cellphone chargers, dvd systems, or small LED TV's. A fan for instance, draws slightly more if the alternator rotates faster, due to alternator 'charging' the battery with a minimal voltage increase at higher rpms. So the fan would receive a higher voltage too, causing it to be able to rotate faster, moving more air, and thus drawing slightly more power."
This is not correct. Alternators use a voltage regulator to keep the output voltage at or below around 14 volts. The regulator limits the amount of current flowing through the coils in the alternator (it is an electromagnet, not a permanent magnet, that generates the magnetic field) and by reducing that current can maintain the voltage generated by the alternator to a certain level.
Yes electrical load on the alternator WILL effect fuel consumption. It will probably be a very small amount unless you are running accessories that pull a lot of amps but it will have some effect. If it didn't effect fuel consumpion as "namgootta" states where is the energy coming from to run these accessories? You may say from the alternator, but what is powering the alternator? GAS. I've even read on other fuel saving forums of people using deep cell batteries and removing the alternator belt to save fuel then recharge their deep cell battery using household electricity. A/C is the largest killer of fuel economy because of the extra load it puts on the engine. If you'll notice when you turn your a/c on the idle speed picks up to compensate for the extra load.
A/C turns on a MECHANICAL pump! The electrical is just for the clutch electromagnet and the fans! I won't deny that it will impact your MPG a bit, but its going to be a tiny portion of what the engine has to do to turn that compressor with the belt as it mechanically makes the refrigerant colder. So if you engage the cooling or the defroster, the pump kicks in and loads the engine via the BELT, not the alternator. I can guarantee you, that if you can make all the electrical parts of the A/C system run but NOT have a compressor installed, the engine would most likely not even flinch.
NO one has a completely electric driven A/C compressor in their car. They would have had to add one from something like a fridge that can run on 12V.
I'm sorry, but the whole basis on this, especially when centered around Air Conditioning, is flawed, pure and simple.