Does the wattage of a power inverter matter if I want to use it to power my laptop?

The "charger" for your laptop is actually a charge + AC adaptor... it will charge your laptop, and it will also let you run that laptop from an AC supply. The 90W rating means that, under normal circumstances (eg, there's no gross damage, the laptop is not sitting in a pool of water, etc). the maximum power draw for the laptop doing anything will be 90W. So if your inverter delivers an honest 90W or more, you're good.

I'm not sure where 240V@1.5A comes from.. that's 360W... but your laptop isn't powered by 240V, it's powered from something in the 15V-20V range. So that figure seems bogus to me.

Typical power maximums for laptops these days run <= 70W for 15" and below displays, <= 90W for 17" units.. but basically, you can trust the AC adaptor the laptop came with.

So in conclusion, your 140W inverter should be fine. You will need a car outlet that can support a bit over 90W == 12V @ 7.5A.

If the laptop needs 240V at 1,5 A, that will be roughly 360W.

This is much more than the 140Watts your converter can offer.

I believe that the 90W charger you are talking reffers to simply CHARGING the laptop battery. I mean, it just recharge the OFF-LINE battery, that you subsequently put in the laptop and use the computer on battery power. Using the computer on outlet power inside the car will need at least a 400W converter. The bigger, the better. On the other hand, 400W will mean that it will suck from the lighter outlet somewhere in the vicinity of 30Amps. That' s huge! your lighter and car battery will definitely have a rough time doing that. To make it more graphic, it would be like having all your lights on most of the time. Or about half the current that the starter sucks from your battery to start the engine. Check that the lighter outlet can handle that output or else you may experience a broken fuse on the lighter. On the other hand, you can have a service station with a good electrician install a power converter permanently in your car, that will supply you with a 220V outlet on your car's front panel. A switch to turn it on/off will complete the works. Since I often use my notebook on the road I've done that in my car already. Good luck!