A more efficient way to power a laptop from a vehicle battery is to use a
d.c. plug designed for that. I bought a Kensington universal laptop dc power
supply for about $50 from buy.com, but you might find ones cheaper on eBay
or elsewhere for less if you get the model for your specific computer. The
one I have has ac, dc, and airplane connections. That way I go from dc to dc
without the inefficiency of the conversion.

Ed