I have an inverter in my vehicle for my tablet. I that know it uses 5v and my vehicle is a 12v system. My inverter has a usb jack and also 2 3-pronged US sytle plugs. Its a fairly beefy one rated at 750 watts continious. Its good for power tools on a job site without a good outlet. I also use it to power my tablet on camping trips.
I bought a cheap Chinese volt/ammeter to moniter my draw. Turning on my inverter costs around .5 amps. Plugging in my tablet costs another .25 amps (on a 12v scale). So, most of my draw is from the inverter doing it converting.
I am wondering what the base draw would be to just have a dc to dc voltage drop? I can seee on Ebay that they are fairly inexpensive but I wonder how they work (guessing they do the switch on and off thing really quick to immitate ac so you could use a transfomer? And of course you could just put a resister of the right size to drop it down, but I would guess that would be the least inefficient.) My main question is though, if is it would be less of a draw than than the inverter. And, if so how much?