How to measure inverter efficiency

[latter]

set up
12v bat. to 150w inverter to a tower pc.
so i put a amp meter between 12v bat and inverter .times the volts by amps get wattage?
then get the wattage of computer using a mains socket(because the inverter produces modified sinewave and my wattage meter may not work correctly with modified sine wave)?

when i have both wattages DC and AC i can work out efficiency of inverter.is this correct.?
so far my computer plus lcd just in idle , desktop is on but i am doing nothing is 52 watts
the wattage between 12v bat and inverter is 66 watts on 1 inverter and 72watts on enougher inverter i have .
i rough idea on this is 30% and 40% efficiency .i find this not good if i am right.
does any know how to get the most efficent setup.
my computer with a wattage meter on. Goes when in use for games from 52watts up to 8o 90 100 watts it is hArd to tell because of the speed of change.i have been told this could be higher but my cheap meter will average out the high watts readings.

should i have a 150watt inverter or 120 watt one.?
inverters always say 90% peak efficiency but don't say what load this peak efficency occurs at.i have been told on a 150 watt inverter you take 15watts off that's the 10% loss
the leaves 135watts so the peak efficiency would be at 134 watts.with a curve going down from there is this correct?
any answers would be helpful
thanks

 

[yungman]

Can you use a more predictable load like a 100W light bulb? Measure the current drawn and the voltage at the output and back calculate.

 

[latter]

yes i could but that wouldn't solve it. but now i think about it are you right because i know bulb is a 100w wow never thought of that idea.
but this would bring me back still to the other problems is my method correct and if i find out as it seems i have 30 to 40% inefficiency what is going on here.I can not afford to keep buying inverters at £30 just to try out its efficiency.
say i am running on average between 50 and 100 watts what size inverter should i look at for efficiency.of course i can get a bigger one but what's the draw back.?
the reason i ask is my method correct is i seem to get high inefficiency results 30, 40 % inefficent.
that is inconsistent with genneral claims of 10, 15% inefficiency for modern inverters