Cigarette Lighter and USB Adapters

[Marc Boucher]

Just noticed that the rear cigarette lighter stopped working in my 1990 100.  Lighter is good, ground is good, but there's no power.  Fuse is good.  Was wondering if anyone had experienced failures with this and knew the likely failure locations and/or repair.  (Preparing for some holiday road trips...Sister-in-law needs to keep nephew entertained/distracted with movies on long drives).

On a related subject, this may be a little NAC, but I'm currently shopping for USB adapter for ipod/iphone/ipad, etc.. You can charge an iphone on as little as standard USB, ie 500mA.  On the other end, the max charge for an iPad is 2A.  And there are some lighter adapters that have two usb slots.  Thus you can get adapters that put out 

as little as 
500mA x 5V = 2.5 Watts

or as much as
2A x 5V x 2 USB slots = 20 Watts.

The question I have is, would an adapter that's plugged into the cigarette lighter draw power based on what it can supply, or based on what is demanded from it?

ie

assuming I've got the dual USB adapter, which can output as much as 20 Watts, but I've got a single iPhone plugged in, which can draw a maximum of 5 Watts, is the draw on the car's electrical system going to be 5 Watts, or 20 Watts?  


Thanks
MC