So, I had a look… and it took a while. There are tons of inverter / chargers out there, along with bazillions of battery options. But here is where I landed if I needed to build a low-wattage but very decent UPS for long runtimes at low power, such as my home heating system (total load < 200W).
Total cost is about $1200 once you account for the various BOP parts - proper fuses, NEMA 15-P Cables, and so on - as well as taxes. But, you get about 3,600Wh of battery capacity and a standby load of 6W. That makes it about perfect for low-load applications with minimal standby.
A comparable-Wh-capacity Ecoflow clocks in at $1,700, with the prime sale going on right now. Unlike the Victron rig, the Ecoflow is a portable product capable of powering far greater loads than the Victron Rig I describe above.
But as we all know, running high-output inverters at low outputs usually really hurts conversion efficiency. Hence the Victron aimpoint of using about 1/3-1/2 of the inverter rated output to power the heating system or refrigerator. That’s what would get me to the rated 91% conversion efficiency.
Notably, some Ecoflow models also work with NUT out of the box, which is a great plus. The Victron approach would only buy you a long runtime and then it’s lights out. Perhaps someone has come up with a way to talk to the Victron (it supports RS485, so it seems plausible) but as solutions go, the Victron approach will take more time and patience to set up than the Ecoflow.
There is a lot to say for going with Ecoflow, such as its portability. i.e. take it somewhere, charge it, bring it back, etc. and that reason alone might make it a better product for emergency situations. A 300Ah battery is no joke to carry around.
Either way, some careful planning should guide your approach to ensure the system is as performant as you want it to be.