Rotary converter effiency depends on how well they're built.

I built an ammeter into mine, and when sitting with no load the needle is barely above zero. The noise of the idler motor does annoy me, although that's probably more to do with it sitting on top of a bit plywood with minimal vibration damping!

I am however considering upgrading to a digital convertor, as the latest purchase is going to be far too much hassle to convert to single phase, and may be pushing the power limits of the current rotary.