https://secure.www.upenn.edu/computing/resources/category/hardware/article/computer-power-usage

You can see that the older computers were over twice as power hungry.

I'd caution that if you're using an Nav program or other processer intensive applications, your use isn't quite as low.

ALSO- Those USB charging items that are using 2 amps or so each (until they suck the motherboard dry) are adding use. ;^)))

The LED displays with variable brightness are fine (we have a NaviComp unit which is wonderful) can't be set that low when you're really working on them in most light conditions (except at 0-dark-30).

Having said that: Don't expect your cigarette lighter 12v to really do all the work if your batter is down. It's take a long time and may not charge it properly. (In your car, you're output could well be well over 14 volts.) If you have a inverter (dedicated or otherwise), I'd use that for a bunch of reasons -- including that it provides you a measure of surge protection, isolation, and voltage regulation.