One reason it's hard to answer the question right now is that, while an engine is capable of producing much more energy than it uses to cruise, it doesn't necessarily produce that energy at all times. Thus, plugging in a gadget adds a bit of load to the engine, but probably a lot less than running the headlights, heater or defogger. So if you drive a normal vehicle powered by an ICE, keep plugging that iPhone into the wall.
The trick comes with newer vehicles that have brake energy regeneration. In these cars, the difference might actually be negligible because they use an intelligently controlled alternator that only charges the car's battery when the car is slowing down and it's the battery that supplies juice for your gadgets. Thus, the engine isn't taxed with spending fuel to keep the battery charged. Over the next few years as regenerative braking becomes more common, getting some gadget energy from your car will at least help reduce your home energy load. Of course, if you're driving a plug-in electric car, fuhgedaboudit. Sam Abuelsamid contributed to this post. Thanks to Ann for the question.
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