"Ian Stirling" wrote in message
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In uk.rec.cars.maintenance Slurp wrote:

"aussie bongo" wrote in message
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snip myu comment
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

Err, no.
If it's measured at 240V, you've then got to addin the efficiency of the
inverter.

Err, no

you have to add in the INefficiency, ie if 80% efficient then add in 20%

If it's measured at 12V, you don't.

????? The extra current due to the inefficiency of the conversion has to be
supplied from 12V!!!!