The authors "92%" efficiency calculations will be with the inverter and power supply at high load.
When drawing only 20 watts, I expect you'll see more like 80% efficiency, and maybe as low as 50%.
When drawing only 20 watts, I expect you'll see more like 80% efficiency, and maybe as low as 50%.
For the AC side, it's much harder to measure - typical inverters have rather imperfect AC outputs, and unless you have a rather expensive multimeter you won't get an accurate power measurement. A kill-o-watt will probably be okay for a rough measurement, but there might well be a +- 20% error...