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Typical laptops consume about 45W of power. The ones that use a Pentium4 or other non-mobile chip (and Intel warned them not to build laptops with that chip) will burn 75W or more. So if your laptop doesn't use a mobile chip...you've got a problem.
The DC-DC power adapter is a MUST, as others have said. Try Kensington or Targus, and sometimes you can find them on eBay for $30. They are also sold as "air power" adapters, same adapter is used on commercial airplanes.
If you go into the power management features on your laptop, and set it up for maximum power conservation, you can cut your power consumption in half. The laptop can and should be powering down the hard drive and other drives, and if you have accessories you can often manually power them down as well. Some laptops let you manually shut the screen (certainly if you close the lid!) but unless you're interactively piloting the boat...dim the screen (cuts power to half of what is used with a bright screen) or shut the screen down (cuts entire power consumption in half).
There are many ways to skin the cat.
Bottom line...for what real paper charts cost? You can buy a dedicated deep cycle battery for the laptop. A Group31 for $125 or so should give you 100AH of power, and that should give you 20+ hours of laptop run time using a DC-DC power supply. That's cheaper than almost any other option.