Just read the back side of the AC charger that the laptop came with and it will tell you how much power it needs on the DC side. In my opinion, a DC/DC converter would not be a good choice because it can't regulate the current flow to charge the Lion battery in your laptop without risk of fire or explosion and you would have to jury rig a special connector. A 120vac inverter would probably be your safest bet running off the accessory outlet with your laptop's charger plugged into it.

My Dell uses 19vdc at 3.16 amps or about 60 watts when the battery is in need of a full charge, somewhat less when it's at full capacity.

hope this helps.

Ralph