I’ve been running my stack with a pair of Cyberpower 1325 VA. All told I sit around 750 watts average use which with the two evenly balanced gives about 10 minutes runtime. Enough to shut things down (includes keeping the poe powered APs up to access things via laptop) with the NAS being configured that if the battery gets to 5 minutes runtime it will shut itself down in case something happens when I’m not around at least the main datastore won’t abruptly crash.
Can find them for about $150 USD each if you look around. If your servers have front displays you should likely be able to see the draw from there, but like mentioned by another a ‘kill-a-watt’ is nice to calculate all those extra bits like a modem or switch that are less obvious amounts.