I've mentioned it before, and I'll mention it again. Some people don't take the phrase "idle consumption" literally. Its ONLY at idle. So as soon as you have a load greater than 1w, you are no longer at idle, and it is considered Efficiency Loss.
So if the inverter is a 60w idle consumption, but you turn on a 100w load, and you have 160w coming out of the battery, thats a 62.5% efficiency. Its not that you have a 60w idle consumption, its that you have a piss poor efficiency at low loads.
This is also why its very important to size your system for what your most common load is. If your loads never exceed 3kw, and your average load is 1kw, then it makes no sense to get a 15kw inverter.