does the dell power up in your house.. I know it's an obvious question..
if it does.. then it's quite simple. it's an issue with the inverter.. 300W should be good enough .. I ran a full PC of of 300W once.. 15" screen and all .. of course you had to turn on the PC first.. wait a minute and then turn on the screen.. much like they had to do in apollo 13 :-)..
anyway .. I recommend dc-dc anyway.. you get nice shutdown controllers and battery rundown protection ..