Silent Generator works.
Step 1: prepare the connector. If you can’t procure one of the connectors, and they’re probably proprietary, then you could carefully stuff some 1/4″ spade connectors into the connector on the back of the UPS. Probably. Might have to make a tool, or use a small screwdriver or something like that. But anyway, strip the wires, trim to length, and then attach the connectors.
Feel free to cringe, because I used connectors for the wrong size wire. I also soldered them instead of crimping, but since they were the wrong size they weren’t going to crimp right anyway, and then especially given that I used solid wire instead of stranded. Potential Fail all around. But all is well that ends well, so:
Then all you have to do is wire it up (the long white wire connects the two deep cycle batteries in series to give the 24 volts that the UPS needs, and then connect the other end of the new cable to the batteries.
And there you go. In initial testing it ran a 200W incandescent light bulb for ten hours before I got bored and turned it off. The batteries still had gobs of charge in them.
Speaking of charge – the UPS will recharge the batteries when the power comes back on. It’s a trickle charger and will take a couple of days to top them off, but it’ll do it. I think in a power outage it would be faster to unhook the batteries and do a quick, partial charge with jumper cables and a vehicle. There’s some anecdotal evidence that my Prius will provide 12V at about 40A, max, with the A/C and headlights turned off. The 201V battery in the trunk, and the engine-driven, three-phase alternator that charges it, can provide 21KW of power and requires requires a really big UPS as an inverter.
Speaking of “Silent” – it won’t be until you crack the UPS case open and remove/disable the beeper element. Otherwise you get a warning beep every 35 seconds or so, which is worse than listening to the generator.