Originally Posted by DKB_SATX
They had an on/off switch and one drew over an amp when off? That's HORRIBLE. Do you happen to remember a brand name? My Xantrex ProWatt 600 claims 90% optimum efficiency and "< 0.6a" no-load draw, but I've always assumed (without testing) that it's essentially 0 draw when switched off.
It is very rare for an inverter to draw any power at all when the switch is in the off position. Some may draw a few mA to keep the circuits which actually control the on/off function hot (so small wires can be used and small switches can be used) on the on/off feature, but my experience is that few even use any to do that.
The thing which draws power is the inverter on, but nothing connected to it, or nothing actually using any power on it. That idle draw can be quite low (60 mA or so) to really high (1.75 A). The factory 1000 watt Sine Wave inverter on my 2014 FC 20 has an idle current of 1.75 amps, on but doing nothing at all. The modified sine wave 1000 watt inverter in my '74 Argosy only takes 0.4 amps when doing nothing.
Inverter idle current is all over the map, and cannot be known unless the manufacturer states it in their literature, which may or may not happen. I have measured small inverters (under 100 watt output) with idle currents of 60 mA (.06 amps) to 0.6 amps. Pure sine wave inverters generally (but not always) have higher idle currents than modified sine wave ones).
Some sine wave inverters have a "search" function which shut them essentially off when there is no output load, and switching them on automatically when a load is detected. There is some power used for that search function, but it is quite small, in the range of 15 to 20 mA. I am going to install one of those in my FC 20 this year and use it mostly rather than the power hungry idle factory inverter. I will leave the big one in place for the microwave and toaster I like to use in the morning but use the other one for smaller loads, and leave it on search function all day. The one I have in mind has a 300 watt output, and will not do the microwave or toaster.