How many watts do I need from an inverter to run my laptop computer? Most of the ones I see are rated at 50 watts. I have a Compaq Presario 1030, CD Rom, etc. I want to use an inverter in my car to power the laptop for use with GPS. Compaq Tech Support says I need 240 watts, but that doesn't seem right. I am totally confused. Will one of those $50 units from Circuit City do the job, or do I need one capable of 250 watt output? HELP?