The inverter itself will draw it's share of currennt, even with nothing
plugged into it. So you need to take that into account also.  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 purschase a
unit that is rated at 25% over what I needed.
     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/