Yes Ebb, your concern is valid.
1. The power draw on idle - I haven't measured it yet but it's probably wise to keep this thing off the batteries when not needed. It might also emit some RF noise. And I'd rather keep an eye on it when it's connected in case it starts burning or something.
2. Power loss when working - The specs state 'up to 85% efficiency' which is of course a pretty idiotic statement since that would also cover 0% efficiency. This inverter is probably not the best one on the block when it comes to efficiency. That said, it didn't even get warm when I ran my power tool (and the tool got *plenty* hot). Which is a good indication that it's efficiency ought to be around the stated 80%-85%. For my intended use it's working well, and cheap.