As the title says. There is an offset always. Example: Zero load (unplugged) and it reads 4W. 4W load and it reads 0W. ECO mode can’t work properly, since I have a load of 4W (refrigerator light). I checked the app to calibrate it somehow, but that is not available.
There is always a small error in power measurement at low powers. Usually this is due to small residual ac currents in capacitative loads &wiring. Generally these are insignificant in the long run.
ECO mode will not be able to detect a load as small as 4W.
It must be a capacitative load and wiring inside the inverter since the app shows 4W without an AC plug inserted into it. I wonder why it can’t be compensated for that.
Also, you need a minimum load of 8W to get ECO mode to work, because the power reading has a negative offset of 4W.