The inverter itself will draw it's share of current, even with nothing
plugged into it. So you need to take that into account also, I would hazard
to guess about 1.5 amps.  The amount depends on it's rating i.e. a 1000 watt
will draw more than a 400w , at idle. I would 'assume' the efficiency would
go up the closer to the rated load you run. I say assume because that
applies for large, entire building, commercial units.
    If it was me, I would do some math beforehand, and try and purchase a
unit that is close to what I needed, say 25% over.
     In this case I agree with Dennis, the twelve volt trucker fan is
probably the ticket. Coleman sells a couple D cell powered tent fans that
are worth looking at. Wally World carries them.
This one looks like it could be easily converted to twelve volts.

http://www.coleman.com/coleman/colemancom/detail.asp?product_id=830-450

Ken Lewis
http://neksiwel.20m.com/