I have a 12v to 19v car laptop adaptor,(DC to DC) which was shop bought
for Eur40, (made in China). It turns ship's 12volt supply into a
selectable voltage to suit a range of laptops. As it is onboard and I
am not, I can't give precise detail now, but do a Google search for
"Car laptop adaptor" and it will link to suppliers.
This is more efficient as it saves energy by not having to use an
inverter to go 12v to 115/230v to power the laptop's power brick. A
win-win situation as it saves the inverter losses in addition to
removal of another noise souce
Noel