Ed is right. Taking 12 volt DC from your battery, converting it to a poor excuse for 110 volt AC, and then having your laptop's transformer back to DC again is very inefficient. Get a 12 volt power device to run your laptop and all the in car cell amps I have seen run on 12 volt DC anyways. Much more efficient overall.
Also be sure you fuse everything. I have been running in this fashion for the last 5 years and it works great. A Powerpole Power Distribution devise like a RIGrunner 4005 to serve up the 12volt are also the way to go. I have a fuse directly off the battery, have run 12gauge for both the pos and neg directly to the RIGrunner in the rear of the car. The RIGrunner has a fuse for power in, and each output is fused as well. You will never be disappointed with this method.
RIGrunners come in many models, but most chasers will be fine with the 4005. There are cheaper models, but they are not fused. If you have ever seen the results of an electrical fire, you will always us fuses. Pairing the correct gauge of wire and the right amp fuse correctly for the device is paramount to not blowing fuses. And never run multiple devises on one fuse.
FYI, I get my RIGrunner products at my local Ham radio outlet.
Also, you can trust Gordon Spencer when it comes to selecting the right amp for your needs.