I have slightly diff requirements from yours.
my aim is to power the DreamC. from the boat's 12V house battery bank. I'll be using a small dc-dc down/buck converter to say 6-7 volts and feed a LM7805 type linear regulator to get the best ripple/noise free 5V power to the DC.
This setup should be relatively efficient and waste very little energy.
Furthermore, I intend to run the DC only once a day, for the required amount of time, to just download the weather GRIBS.
Bump.. any OuterNet support guys able to comment?