I have a laptop with a short battery life that has a proprietary high amperage plug that I need to use in a car while that car is in motion, being driven by someone else. The proprietary plug rules out a standard DC to DC converter to power it, and I have tried, with no avail, to find a compatible DC to DC converter. As such, my only option is to use an inverter. A large inverter, that connects to the car's battery directly is too complicated for me to feel comfortable installing, especially considering the high voltage involved. It is also too expensive to have it professionally installed. This leaves products that provide a 120V house outlet from a cigarette lighter, such as this. My laptop's power connector, however, says that the input is "100-240 V ~ 3.2 A 50-60Hz" and that the output is "19.5 V 11.8 A". I am aware that volts multiplied by amps gives watts, but because there are ranges for the input voltage, I get an input wattage of anywhere from 320W to 768W. If I use the output information then I get 230.1W, although this is likely less accurate with regard to the actual power consumption due to transformer inefficiencies. Two of these values are less than the 400W that the linked inverter says it is able to provide, but 768W is far above it. How can I resolve this ambiguity to know how many watts I actually need to run my laptop? What happens if I make a mistake during that resolution of ambiguity and end up with my laptop trying to draw more power than the inverter can provide? Will it just blow a fuse that I can reset (and then not use that laptop with that inverter again), or does it have the potential to damage other things (ie. shut off the car while it is in motion, or blow a fuse deeper inside the car that is difficult to replace with my next-to-nonexistant knowlege of the inner workings of cars)?
Some other questions give useful information for part of this question, but do not address the car and inverter specific factors, such as what happens to the car and/or inverter if the estimated usage ends up being too low.