I picked up two Eaton UPSes, a 5S700 I planned to use to power some office equipment and a 3S550 I plan to use to power my networking gear, both indeed handle the 66Hz output by the Powerwall without issue as expected (though it seems my office survives the PW transition just fine on its own, so the 5S700 might end up powering my entertainment center since my Vizio TV shuts down when the PW kicks-in, whether the PW is charging or discharging when the outage occurs). But what I didn't realize is that they're much pickier about proper grounding than any APC/CyberPower UPS I've used. My 1960 house before I bought it had most of the outlets replaced with 3-prong even though they're not grounded, and while other UPSes ignore it or at worst light a "wiring fault" LED but otherwise function normally, the Eaton ones outright refuse to turn on if plugged-in to an ungrounded outlet, and remain on battery if turned on before they're plugged-in. I wound up testing them in my kitchen where all of the outlets I installed in a 2009 remodel are grounded, but any of the locations I'd like to deploy them to will need to be re-wired first. Not that it's a bad thing, but it's an unexpected nuisance that wasn't a restriction for other UPSes I've used. It also made me think the first one was DOA until I realized why it wasn't turning on.
While the networking gear survives the outage, my real plan was to add DC-DC converters running off the 3S550's 12V battery, to save space on bricks/wall-warts and outlets needed for a structured wiring cabinet I plan to do.
