Good points John.

Typical rated power to the AC adapter for a laptop is close to 150W, but
output to computer is in order of 65W. Explains why the damn things get hot.

Direct power may well be worth investigation, rather than using DC to AC to
DC.

surfnturf

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"John Weston" wrote:

Watts = Volts x Amps snip
If your laptops are so inefficient as to require the full 300W
of available input power from the inverter then yes, that
equates to at least 25A at 12V. If it requires less power,
then the current drawn from the 12V battery drops to that
necessary to provide the power.

It doesn't matter that the laptop is being supplied at
120/240V from the inverter (and so the output current will be
around 2.5/1.25A), you'll still need more than 25A at the
input to the inverter if it needs the full 300W. If that is
the case, you wouldn't be using a 300W inverter, since they
don't like running at their maximum output for too long.

Typically, with computers, the rating of the power supply is
based on the expected maximum power requirement of the
peripherals and circuits, plus a bit for reliability. What is
actually drawn from the power source depends on what you are
doing with the computer. I'd expect a computer on a boat to
draw a *maximum* of around 100W (depending on its design) so
your cigar-lighter socket should be OK at around 8A. However,
if you ever try to power 3 or 4 working laptops, then your
fuse may blow, if the inverter doesn't quit first...

snip John