I have the 13.5 unit. Not sure why it is surprising. I noted I would not take just one unit if I were off the grid and needed ac, I only mentioned that it did work in my situation. As far as noise, the Champion puts out 53 db; not sure why you think that isn't as quiet. I owned a Honda E3000 and my experience was that the Honda and the Champion were equally as quiet but they did have a different sound "signature".
I was in my shop yesterday working and had one running on a workbench charging a 12 volt battery. It was not noticeable thru ambient noise such as traffic, a fan or the radio.