Personally, I wouldn't run any lighting on the battery backup. Lighting is not an essential life support device, especially since you have a FOWLR tank with no photosynthetic corals. In the event of a power outage, the most important things to run are your pumps to keep water moving.
The unit above is rated at 450w, which means that's about all the power you'd get out of it. To calculate very roughly how long this would run your equipment, take the total number of watts (450) and divide that by your equipment draw. If we assume you want to run HOB and PP8, and we assume your HOB uses 15W and your PP8 uses 15W, then your total power draw would be 30W. If you divide 450W by 30W, you'd get around 15, which is the maximum amount of hours that UPS would run your HOB and your PP8. Obviously you'd need to add in your Biocube's pumps to that figure, but that's the basic idea of how you can calculate runtime.
That, of course, comes with a few caveats. That 450W figure assumes 100% efficiency of the inverter and other electronics inside the UPS. Which is unlikely. To be safe, I would only assume you're getting 80% of the total capacity rated, max. Additionally, assuming the device uses a standard SLA battery, it is not a smart idea to discharge these things all the way to zero. I'd knock another 10% - 20% or so off the figure so you don't overdraw the battery. So if your original calculation says you'd be able to use that UPS for 15 hours and we're only assuming we'll get 70% of rated capacity, that works out to be about 10 hours of total runtime (15 hours * 70% = 10.5 hours).
Personally, I would go with something a bit higher capacity. The problem with these UPSs is that you can't upgrade them once you buy them. You're stuck with that capacity forever. If you find out that you need more capacity, you have to buy another. Either that, or I would DIY a system with an
AC/DC inverter,
battery, and
transfer switch to cut between wall power and inverter power in the event of a failure.
