You likely only need 1350 but a 1500 does not cost a lot more. These are 2 example of a popular brand, note these are both sign wave models.
The main difference is the 1350 is only rated to handle 800 watts which is still more than you can use even if you were running at maximum load. I suspect you are using much less.
The 1500 is rated to 1000 watts.
The other difference is the 1500 has 2 12 volt 9ah batteries where the 1350 has 7ah. What is kinda the scam is in 3 years when you need to replace the batteries you can put 9ah batteries in the 1350 unit, they are the same physical size. It will still only be 800 watts but last much longer.
The difference in cost is only about $2 for the batteries with more power, so I don't know why they don't sell them with that to begin with.
A mini-tower UPS with line interactive topology, the CyberPower PFC Sinewave CP135PFCLCD provides battery backup (using sine wave output) and surge protection for desktop computers, workstations, networking devices, and home entertainment systems requiring active PFC power source compatibility...
www.cyberpowersystems.com
A mini-tower UPS with line interactive topology, the CyberPower PFC Sinewave CP1500PFCLCD provides battery backup (using sine wave output) and surge protection for desktop computers, workstations, networking devices, and home entertainment systems requiring active PFC power source compatibility...
www.cyberpowersystems.com
Note they do actually sell av1500 units that can run 1500 watts but they cost more than double the price.
There are many other brand that will work as good. Just be sure you are buying true sign wave units.
A addition comment. Most better UPS also have the ability to boost the power without actually running on the batteries if the voltage drops just a small amount for long periods of time. If it drops below some value it will then move to the battery.