Measure or calculate the total power being used by your system is step 1.
You can measure with a device like a Kill-a-watt meter.
For your PC there are several website to help you calculated your power consumption based on the hardware in your PC.
Add up all the powered used for the things you wish to be on battery backup.
Then you take that number (PC& TV & all other equipment you will attach to the UPS) and multiply by 1.2 and go UPS shopping for something with greater number in the VA spec.
Example: You calculate 440w total PC power + 188w TV = 628w
628*1.2=753.6 Now this fictitious user would go shopping for an 800VA (or larger) UPS.
When you find one you are interested in, you check the UPS's RUNTIME charts to see how long it will be able to maintain the load (753.6)
I have a feeling you are going to be putting on your networking equipment as well.. You might want to get a smaller UPS just for those.
Lastly, keep in mind that batteries age and you will need to replace them or the whole unit periodically.