In uk.d-i-y "Dave Plowman (News)" wrote:
In article ,
aussie bongo wrote:
the current drawn at 40w on 18v is around 2.222222222222-amps // but
40w at 240v is about 6-amps
snip
So 40w at 18 volts is as you say 2.2. But watts are sort of an absolute
measurement of power, so increase the voltage and the current goes down,
so 40 at 240 volts is 0.17 amps.

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