does anyone know how much energy (watts/amps) your ac draws
ac would be on cold and slow fan speed, the vehicle would not be running while the ac is in use.

 

If the engine is not running, neither is the A/C. The engine turns the compressor. Without it, you just have the fan. It draws a fair amount of current, probably. It may draw the same amount regardless of the speed, since the resistor block absorbs some of the current headed for the fan motor and turns it into heat. Strictly a guess, but I'd suspect the fan circuit draws 30A or so.

 

I'm not quite sure if you're asking if the fan draws current, or if the operation of the a/c as a whole effects mpg. A think a simple answer here might help. The fan does draw current off the battery/ charging system. If the engine isn't running, then its simply draining your battery, which will cost in the end, as current +amp draw, fueled by gasoline to recharge the battery. On the same note, compressor operation, driven by the engine, equates torunning head pressure of the system, plus fan current which effects mpg. In other words---the more work your engine has to do, the more gas thats required to do it.

Can't get something for nothing

 

on a 1995 5.9V8
turning on the AC at idle speed
increases the fuel consumption 0.2 gallons per hour