A car battery for me will run my desktop PC and a 17 inch LCD for about 5 hours before the inverter beeps and it will beep for almost an hour before shutting down, and I only use a 400 watt inverter to run them both, but it does have it's own battery so I don't kill the car battery when it's not running... So a xbox should be nothing for a battery even with an 8 inch monitor... You might also want to look at your output voltage lots of inverters are self protecting in an overvoltage situation and this will kick in some where aroun 15 to 16 volts DC....................

Oh the other thing was, what gauge cable/ fuse did you use? As inverters normally require a high gauge of cable and a unusual amp fuse for a car, 40Amp if I remember correct. Would you have used insufficient cable/fuse for the unit. check that. If you used the inverter on a boost box and it worked then it will work in your car... thats what makes me think its the cable to the batt or the fuse you have used or you have a tiny tiny battery, is it a small cap 4 cvlinder car?

Wait wait wait..... what does the cars 20amp fuse have to do with it? It shouldn't be running through the cars fuse system if that is the case... So you may or may not have an ignition wire a power wire and a negative wire. The ignition cable should be wired into the radio on or the closest ignition on pwr cable (which will obviously be on a fuse somewhere but that doesn't matter) The negative to a ground point on the chassis or ground cable. The power cable needs to be directly wired to the positive on the battery or the pwr cable of the battery prior to it entering your cars fuse box. A 20amp circuit in the car is not adequate to run the inverter. The cable I run is close to a 1cm thick with a 40amp fuse. Check the amp rating in the inverters instructions, it should also be on the base of it. I sell inverters and haven't seen any rated at 20amp for car purposes. Try running the inverter briefly with out running it through the cars fuse box. Remover the cars leads of the battery and wire the battery directly to the inverter, no fuses. Make sure you secure the cable from inverter to battery in a way that allows you to remove it QUICKLY incase things go nasty.. If it runs directly off the battery then its the fuse/cable you have wired it into... Go get some 50amp cable and a 40amp (or designated, see instructions) fuse... It is VERY important you get it right with the amount of power going to the inverter... Just as an example of if you get it wrong (I was aware of thisprior to the event, it will never be done again. Lesson learnt ) Wired an amp up minus the fuse inline to the battery on the car. Cable crossed in the boot and the entire length of cable from boot to battery melted it shielding completely off and stared smouldering , melting carpet and almost combusting the carpet in less than 2 seconds. I pulled of the road immediatley and popped the bonnent knowing if I ripped the power of the terminal the heat source would be terminated. I grabbed the cable and ripped it completley out of the car from boot to bonnet. This left me with about 12ft of still almost redhot cable minus all traces of it once existing insulation. To my horror after the event I noticed the fuel filter in the engine bay had a burn mark on it. With closer examination I found the cable had melted to the mid point of the filter and had made contact with petrol.... One lucky man... Like I said, it will NEVER happen to me again... Now I triple check all of my specks on every occasion.

. Excellent suggestion.
. I doubt if the battery is the problem. Possible, but low on my list. He said the car was running; unless he had the headlights and all accessories turned on, the alternator should be able to keep up. If the alternator is weak, it may have a problem putting out enough at idle.

. With a 300W inverter, you have two things working against you with a 220W load:

• Inverter manufacturers have a tendency to "play loose" with output figures.
• Most equipment has an inrush current that can be several times the operating current.

. Ie, a 300W inverter may be big enough to operate the Xbox, but not get it started.

. Instead of "play loose" I should have said something like: They tend to use best case, on a test bench, not exactly real world load figures, instead of what John Doe will get out of one. . Kinda like size ratings on HDDs, they aren't really being dishonest, just using units of measurement that most ppl aren't familiar with.

. I don't know enough about your inverter (or inverters in general) to swear you don't have two 150W ports (receptacles?), but I'll bet a nickel on it. You should be able to get full power from either port, as long as the combined load does not exceed 300W. If it is 2x150, it should say so on the box.