Its definitely not up to code, but that is not really a problem unless you plan to do a larger (permitted) project in the future or when you sell the house and

Its definitely not up to code, but that is not really a problem unless you plan to do a larger (permitted) project in the future or when you sell the house and are getting it inspected. The one thing to watch out for is that if your house does burn down (presumably due to reasons not related specifically to the non-code-compliant wiring) the insurance company will try likely try to deny your claim. At least in my jurisdiction (America) the only way to achieve what you are trying to achieve is by installing a dedicated subpanel fed by the UPS with appropriate breakers. I believe that such an installation would only be code compliant with a UPS that was designed for such an application (with a hard wired output connection, and appropriately designed disconnects), but a lenient inspector might let you get away with feeding a subpanel from an extension cord as long as as of the breakers and capacity factors work out correctly and the grounding meets code. The biggest issue I see is that the system is not labeled correctly (at all?). That is a pretty serious issue because if, for example, an emergency worker were to come into the house under the expectation that the power to the house was cut (at the service disconnect) but the house would have live outlets that are otherwise unmarked. See https://www.mikeholt.com/download.php?file=PDF/11_Generators_and_Standby_Power_Systems.pdf https://www.mikeholt.com/download.php?file=PDF/11_Generators... for more information (the relevant section for your system starts at page 16) As other commenters have pointed out, just because you can buy something that says NEC approved on the sticker does not mean that it is approved for your application. A good example mentioned elsewhere is using a 16awg SJTW extension cord to power a 15a load--all of the connectors will plug together but the combined solution is obviously not code compliant and even if it doesn't melt/catch fire it will not supply the load with clean power.