For OP's system, or indeed any single home PC, provided it is the only load connected to the UPS - and that the user does not intend to keep gaming on it in case of a power cut - any 700-850VA UPS (420-510 Watts output) would be more than sufficient to have enought time to shut it down properly. I'm repeating above advice but as for the actual PC a good quality 500W PSU is quite enough.
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Below the text wall on UPS:
As far as I know and will try to keep as short as possible. There are usually 3 kinds used in home/office and the 3rd kind mostly used in mission critical situations such as data centers, server rooms:
1- Off-line (or Standby): Just comes 'on' when there's blackout when AC (or utility) is cut. They 'use' the battery during brownouts too and usually offer limited or no protection against power irregularities (over/under voltage, surges and floating frequencies and such) and usually have 95-98% efficiency. They're cheaper than others and smaller in size. If load requires higher than 2kVA these are almost impractical.
2- Line Interactive: They are more reliabile and have higher efficiency (typically 90-96%) and they provide reasonable voltage conditioning. In the event of high/low voltage, surges and floating frequencies they offer kimited kind of protection. For example they have transformers to buck (reduce) or boost (increase) the voltage so they provide voltage regulation to some extent as the input voltage varies.
Voltage regulation is important because when there is low voltage without it the UPS would transfer to battery power and frequent battery use can lead to early battery failure. The range at which these buck or boost voltage is normally limited to around 10% and some models will provide both buck and boost but other less expensive models will just provide boost capability. In other words when there's over voltage they don't offer any protection unless it goes so high so fast it's treated as a surge.
3- On-line (or Double Conversion): There are the most common UPS used in situations that require above 10kVA (around 9 kilo watts with efficiency of around 90%) and are designed to provide continuous power protection against all power problems to mission critical equipment. This kind/topology ensures a consistent quality of power supply regardless of disturbances and irregularities in the incoming mains.
In the double conversion on-line topology, the primary power path to the loads is the inverter instead of the AC mains, thus the inverter is ON 100% of the time, hence the name on-line. The double-conversion part of the name comes from the operation of these devices. They first convert line voltage (AC) into low-voltage DC for the battery using rectification circuitry. The rectifier supplies power to the inverter and at the same time charges the batteries. The DC from the batteries is then converted by an inverter back to the highly controlled/regulated AC. This process thus effectively removes any electrical disturbance or irregularities on the utility (AC input) side of the UPS.
The Double Conversion UPS is the only truly uninterruptible system because the inverter is always connected to the load and is always drawing power from the battery source reagardless of whether the mains power is there or not. Therefore, during an input AC power failure, the on-line operation results in no transfer time. This kind also provides protection against all forms of power irregularity including surges, spikes, over-voltages, under-voltage, blackouts and brownouts . These UPSs maintain a highly closer output voltage tolerance compared to the line-interactive ones as instead of transformer taps they have solid-state voltage regulators to deal with under/over voltages.
Althoug the Double Conversion On-line UPS provide nearly ideal electrical output, the constant ongoing wear on its power components reduces its reliability and the reduced efficiency (around 80–90%) results in higher operation and maintenenace costs for the UPS.
All that said
The line-interactive UPS are kind of the sweet spot between 2kVA to 5kVA. As Off-line/Standby ones are almost impractical over 2kVA and On-line/Double Conversion ones are too expensive for under 5kVA use. For home use an off-line one would do as a good one would provide around 15-20 mins to shut down the PC system properly and the time it can handle of course depends on the load.
For example let's assume a 1.5kVA unit (with 120V input and output) which provies 900W can hold a 100W load for almost 70 minutes. If you connect 4 identical PCs (load) to it and each use around 100W at idle the total load would be 400W and the time the UPS (at good condition with good batteries) can keep these PCs ON would be around 17.5-18 minutes.
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