Most laptops only draw about 60-75 watts. So even a 150 Watt inverter is plenty for that. Just remember to give that sucker some breathing room, I killed one when it got buried under blankets on a trip.
Basic forumula: Watt's Law P = I * E
So Watts = 110 * Amps which is on the bottom of the AC Adapter.
Can't make it easier