What size power Inverter?

[Khanhfucious]

Im going on a road trip soon (that I dont plan on returning)

My Laptop is an Alienware Area-51m 776.

Power Supply is Lite-On Mode PA-1161-02

Input 100-240V 50-60Hz 2.25A
Output 20V 8A

How big of an inverter do I need?

 

[Christopher]

For fixed voltages, you can say that watts = volts * amps.

So, your drawing 2.25 amps at 110 volts (standard US mains power), which is 248 watts.

I'd get an inverter rated for at least 300 watts, 500 watts would be better to give you some headroom.