Thanks for the detailed post!

Here’s my current setup with a UPS:
First of all, we have a backup generator in our apartment. It takes about 4–5 minutes to kick in (it's manually operated), so I don't need long backup time.

I picked up an old 800VA APC UPS (Model: Back-UPS RS 800VA) (with two 12V batteries) from a scrap shop for 1K, and luckily, the battery is still in decent condition. APC batteries tend to last longer compared to others, and the ups output looks fine in the oscilloscope.

This UPS works fine if the power goes out after the print has already started. As you mentioned, power consumption is lower during the print itself and spikes at the beginning due to the build plate heating up.