"aussie bongo" wrote in message
...

"Ian Stirling" wrote in message
...
In uk.d-i-y aussie bongo wrote:

"Lobster" wrote in message
...
I'm thinking of buying an 12V - 240V for the car, to enable me to
charge/run devices such as a laptop / mobile phone(s) / AA batteries
for
gameboys & camera etc etc while on the road (one at a time!); they seem
to
be as cheap as chips these days and this would enable me to use the
standard mains chargers for all the above devices rather than buying
separate low voltage kit to connect each of them up to the cigar
lighter.
snip
i dont know about power usage on a laptop, but,
i do know that 100w drawn from an inverter at 240v, will give you less
than
1/2hours usage.

YLMV.

And 100W can never give any meaningful amount of run time without
specifying how long.

My laptop - admittedly a subnote - uses about 40W when charging, and
about 15W running normally.

This runs on a standard car battery for around a day, from an inverter.

but is that 40w &15w at 240v or the normally 18v that most laptops use??
this makes a big differance


40W is 40W whether its at 1nV or 306MV
P(WATTS) = VOLTS x AMPS

40W from an 12V battery = 3.3A
40W from 240V rms supply = 0.167A rms
Average car battery = 35Ah, assuming 100% conversion efficiency = 10hrs