Dave Plowman (News) ) gurgled happily, sounding much
like they were saying :

Thus the wattage drawn by the invertor will be the same as its load,
but increased by the efficiency figure of the invertor. So if that's
say 80%, the load of 40 watts will draw 50 watts from the battery.

Plus the inefficiency of the laptop power supply, which may be another
20%, or maybe 60-65W

The 40 watts was just following on from ab's post - not meant to be an
actual figure for a laptop.

60-65 watts might be closer when it's charging the laptop battery - the
actual running current will be very much less.

Depends on the laptop.

Mine (big widescreen) whinges if it's on a 65w psu, and came with a 90w.
Others come with a 45w psu.

I've never come across a laptop psu that wasn't clearly labelled, though...