I guess I'll share the little bits I've figured out on ferro-resonance UPS's.
The large transformer favors running inductance loads - Since it is rated in KVA note the rated load into resistance figures are probably 30% less or about 12.5Kw.
From the Man'f label photo - If you multiply out the AC line input and subtract the output specs it suggests losses of 416 watts at 208V and 720W at 240V. My bet is when it is online and the batteries are charged those are the parasitic & constant wattages the unit consumes to keep the controls and transformer resonance alive and ready to perform using pass-through AC.
If you need the heat somewhere just leaving it plugged in would make a nice little space heater. Also note is has very little margin between maximum rated load and running overloaded, that transformer probably sits at 120°F just sitting connected doing its line-conditioning service, and the units surge capacity may only be 10% or 15% over its rated output.
DC-input to AC-output is more efficient, the losses are still there but the unit is no longer doing 'line conditioning' on the raw AC, similar losses are there but not as great. People who are lucky enough to have 'excess' RE energy could certainly use the unit as is for a dedicated power source - running it a few hours to power shop tools or pumping water etc. the losses might be acceptable.
