On grid or off grid, you still have to meet the code the AHJ enforces. If the MG set (electrically driven generator) is connected to the grid, it is a grid tied system anyway, just a separately derived system, similar to going through a transformer. Make sure you follow the SDS rules, mostly grounding issues. As for the listing, or lack thereof, you may be stuck if you are not willing to have it evaluated. My guess is new equipment might end up being cheaper than that process. First be sure it was not listed by one of the European NRTLs recognized here (TUV is probably the only one) The CE mark is pretty meaningless in this discussion. It is a totally different standard.
I am not sure how field evaluation works but we had a guy at one of our contractor seminars who was making some of the first generation salt water pool chlorine generators and he said U/L was horrendous. They kept blowing up his prototypes and asking for more. This was destructive testing. I assume field evaluation of existing equipment is not like that.
Greg: RE: The field eval mentioned in my above post...
It was three packaging machines, for a large on-line operation. These exact machines were previously installed (and field tested) at their other area locations. Three (3) locations in an adjacent town.
The field evaluation was a multi day operation, and labels were affixed. Units were 480 volt, 3 phase, plug-in, and 120/20 amp plug-in.
I was not present during the cert, but got copies to put in the job files. Foreman said it was "painless".
Field tested by a NRTL? I suppose a lot depends on what they are testing. If this is assembled from U/R parts and they are only looking at the assembly methods it should not be that complicated but if the actual equipment is foreign and never evaluated at the component level there is no way the field test can be as extensive as the lab test. I have to say, I have only heard people talk about this. I never actually watched it. OTOH, under a lot of pressure, I did certify a lot of stuff at the Ca D Zan (Ringling Mansion/Museum) in Sarasota without any protocols in place at all. This was antique equipment and for the most part I just assumed if it had not burned up in the last 70 years, it would last 70 more I mostly was looking at the condition of the insulation, security of the attachments, chafing and other hardware issues. A lot of this was restored under supervision of the engineering firm the state hired. When possible they used listed components and modern fixture wire but the things you could see were original. The guy I worked for in Tallahassee just told me "make sure it is safe" when I asked him what my job was. It may have been the most interesting thing I have done in any of my careers.