DIYbanter - View Single Post - Powering laptop etc in car from inverter?

In uk.d-i-y wrote:

Ian Stirling wrote:
In uk.rec.cars.maintenance Slurp wrote:

"aussie bongo" wrote in message
...
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

He did. Which bit of "assuming 100% conversion efficiency" don't you
understand? It might not be a valid assumption but the method is
perfectly clear.

Indeed, and my point that he completely missed the point that asking
"Was it 40W at 18V or 240V", is obviously referring to the conversion
efficiency.