Most power outlets in cars will give you enough to use an inverter that gives around 250W. So just make sure your inverter supplies enough for your power supply for your laptop. Additionally, you may want to have some spare fuses, just in case. Oh, and if you can get an inverter with some active cooling, get one, those things can get hot. Make sure to get an inverter that can supply 25% more power than you actually need, remember that a 110W power supply might actually draw more like 120-130W from an outlet, and getting an inverter that can supply more than you need will ensure that it can remain cool. Finally, make sure the inverter can supply the power you need non-stop, I've seen plenty of inverters that say stuff like "250W power inverter", and then in fine print it says "250W for 5 minutes, 200W continuous".