Just looked the thing up and it says it only needs ~1 amp @ 120V which is really really low.
If you do the math from above W = V*A you end up at 120W required @ 120V.
Something like this should work great:
http://www.homedepot.com/p/Power-Br...ower-Inverter-XR400-12/202539660#.UcT9EvnVDh4
What's important to realize is that potential difference (voltage) drops as a battery expends its potential energy (stored as chemical energy). This means that the voltage supplied drops which ultimately means your wattage drops.
Are you sure your car battery is charged all the way up? I think this is ultimately the problem you're going to run into.
The person who said a friend used it in a car makes sense, because the alternator in a vehicle would keep the battery constantly recharged which prevents voltage drop. Typically voltage drop is the reason why jumps are required in a car. This is why it especially happens in winter a lot. As temperatures drop the kinetic energy of electrons also drops and this reduces potential difference.