Honda gens are pure seine wave I am sure there are others but most are just modified. Proportional systems are not likely to work properly with modified seine wave. On off ones will however.

there are some UPS systems that are pure as well but generally they will not power any heating system for long enough to be useful.

OP I suspect the battery unit you are looking at is similar it is likely a short term option. I have looked at a back up system and found that 2 or 3 deep cycle marine batteries will give my system about 12 hours or so.

The option I have investigated is a DC inverter with a transfer switch build it 3 deep cycle batteries and a trickle charger Much like the system emergency lights run on. Power goes out and the system powers up the back up system. It is independent heat and t-stat (set to 85) and when the enclosures drop down to this point the s-stat kicks on I have about 12 hours of 50% usage in reality I will get a bit more as my system is very efficient and I should get a bit more as i usually run at about 10%.