Is the UPS supposed to match the wattage of the power supply inside my computer case? In other words, if my computer runs on 850 Watts, is the UPS system supposed to be rated for 850 watts? How does this work?
I'm no expert but it's probably a good place to start. I run mine from a 650w one (older, with new batteries), with a 750w power supply. My components use 414w (est.) plus my two monitors and my router/modem. I have tested it and it all ran for about 3 minutes just fine on battery with 3dmark running at which point I shut it down and plugged the UPS back into the mains. I never leave my rig running unattended so this is all I need.
Your UPS should exceed a bit the power needed by PC.
Ok, the PSU max power is reached for small amount of times so in practice a UPS matching the PSU Wattage should be enough.
If you force UPS to deliver more power you risk fire due overheating the battery - if it is an advanced model it will sound alarm for OVERLOADING and possibly shut down.