homelab

Other have answered the runtime and load question very well already.

I have three other points.

1. Batteries degrade over time. Over-speccing your UPS means more likelyhood that things will hold up in three years time as the capacity given is for new ones. Plus, not running your UPS at 100% capacity reduces its stress. Again, more reliable.

2. You can get a much better quality UPS by buying a second hand one without batteries off ebay and replacing them yourself, typically for a fraction of the cost of buying new. Plus you know you have new batteries. UPS is something where quality genuinely matters. I've had to carry a cheap and badly made UPS out of an office whilst it was on fire, so now I spec more carefully. (And ensure they're metal bodied!)

3. Consider what you NEED to power. What sort of power cuts are you expecting? Does it matter if something goes down?

I UPS my servers and my main desktop, but not my routers, nor my wifi or IOT things. My internet provider also goes out when there's a cut (I'm on a mesh system so rely on neighbours, who will typically also be down) and I can't do much without power anyway, but it keeps the disks spinning. We typically get very short automated outages here of less than 10s (yesterday was a bad day, we had 9 within 2 hours)

After you know the wattage of your load, use the Runtime tab on the product page to see how much runtime you get at a given wattage. https://www.cyberpowersystems.com/product/ups/smart-app-lcd/or1500lcdrm1u/
The one you have selected will have 2 minutes of runtime ~900w on a fresh battery. If all you want to do is clean shutdown, that's probably fine (as long as you don't have brief power drops that you want your shutdown procedure to ignore). If you need more time then you'll need a different UPS.

Note that some UPS models will do better for a given task than others with similar specs. For example I have a ~100w homelab, and wanted maximum runtime for 50w during a power outage (servers off, router+switch+aps active). Some similar spec Cyberpower models were ~120 minute runtime at 50w while others were 200+ for the same load.

A typical NUC that I know of draws like 10-20W, maybe you're looking at the power supply maximum ratings, and not the actual tested draw?

The thing you're missing is runtime. If you match your usage to the UPS exactly, your runtime will be zero. If you consume 300W, your UPS needs to provide 300W for however long it is until the power is back. All major UPS companies (APC, Eaton, Cyberpower) have calculators on their site.

But a UPS for extended runtime will be very expensive, and lead-acid batteries must be replaced every five years or less, else they will not be able to sustain a load, so budget for that maintenance too.

A UPS is really just to bridge the gap between mains power going out and a generator coming online. A large-capacity UPS will be prohibitively expensive.

Also, note that cited power figures on the NUCs are probably peak draw. They will probably draw less on regular usage; you'd have to measure it to know for sure in your specific use case.