I realised, that these were 2 of the 3 voltages needed to power a PC. Could it be done?
Well yes it can! But straight from the 12v feed.
I found this site:
M4-ATX, 250w output, 6v to 30v wide input Intelligent Automotive DC-DC Car PC Power Supply
We need a very special power supply - namely, the M4-ATX from Mini-Box that can handle input voltages in a range from 6 to 30V DC.
This is supplied as a power board only! There is NO enclosure and NO cooling! So make sure you provide these before you fire it up, or it MAY set fire!
This board provides 250w power! Plenty!
Step 1: What PC Components Do I Attach?
I searched for the lowest wattage PC. Here i found a 25w system.
This does leave a lot of spare wattage free for expansion. But if you are going green, you may as well be as green as possible.