Sorry this topic has been discussed in many forms before.
I need a computer for the boat which I am going to buy ( 45 to 50 ft
cruising sailboat). I initially considered a desktop but was scared off
by the power consumption.
Now I am considering a Laptop. I want to use it to run navigation and
other boat soft ware, but I also want to use it for e-mails and for web
surfing when I have access and also to watch DVD's and CD's, so I feel
I need a Pentium 4 with Windows XP.

There is a bewildering choice of brands. One way to narrow the choice
is again power consumption from the house batteries since the computer
will be on a lot.
I can get an inverter and then plug in the power supply to that. But
that route looses about 20% of the power.
The better option is to have a DC to DC converter like one that is used
to connect a Laptop to the cigarette lighter in a car. But how much
power does that consume?
Third choice, is there a brand of Laptop eg Toshiba or others which
accepts power stratight from the House battery, ie a voltage anywhere
between 10 and 15 volts?