Sorry, I can’t give you an indication of runtime for your situation but I personally run a CyberPower 1500VA UPS for my 65g tank. It will power my Apex, Wavemaker, Return pump and Skimmer for about 90 mins, but I use Apex programming to move to a keep-alive mode to stretch that to about 5 hours.
If you just run the return pump you would get much longer life. DC vs AC will change the life too, my pumps are DC, believe they draw less power than AC.
I have a power inverter from Walmart that I purchased for $35. The equipment plugs into the inverter, and the inverter is connected to a 12 V deep cycle marine battery. Both are located in the automotive section of Walmart. The marine batteries have blue labels and will indicate deep cycle/ marine on the label. Select the largest size ( group 30), and the inverter with the higher wattage ( I think mine is up to 750 W) You can have yourself a battery backup system for just over $100 in this way.
I have run my tank for over 24 Hours, running 3 EcoTech vortex pumps, my return pump, and heater, using a group size 30 battery (the largest marine battery). Additionally, I have a bass boat stored in my garage that has 6- 30 sized marine batteries, maintained on a triple charger, that I can pull from if required. In this way, I can run my tank almost indefinitely using the batteries.
The downside is that this battery backup method will have to be set up manually; it will not turn on automatically, and in the event of a power outage someone will have to be present to set this up. Otherwise, it is an excellent option, perhaps as a solution to be used in combination with a UPS after it runs out.
If you google around, most UPS manufacturers have a UPS sizing calculator on their website, but in general it depends how long you need it to run for. As a super rough estimate, figure a $100 700VA-850VA UPS would likely run your pump for 3 hours or so.