How does a UPS work?

let me see if i can give you a easier to understand and more thorough answer.

as far as what can be hooked up to your wall outlet that does indeed have to do with your house wiring diagram. typically anywhere from one or two to as many as all the outlets in a room are wired together and will terminate at the circuit breaker. the wiring in your house may be able to support 15amps or 20amps and this is the size of the circuit breaker in the main breaker panel. all of the devices will need to draw less than this total or the circuit breaker will trip causing you to lose power. this is done for safety so you do not melt the wires in your house. i found a source which ways that a 20a circuit can support 2400watts maximum but should only be ran on 80% of that number or 1920w

if you have a single circuit connected to the breaker this means that you can pull that from the one wall outlet however if you have multiple outlets or even the ceiling fan hooked up to the same circuit (depends on how well the person wired your house) this total is spread across all of the connected devices. this is why in kitchens outlets are often either on their own loop or placed in small clusters since many devices (microwaves, blenders, tabletop convection ovens, etc) draw quite a bit of power and shouldnt all be ran on the same circuit at the same time.

if you want to know how many amps are being drawn perhaps you should get a kill-a-watt which will tell you exactly how much power devices draw.

as far as how much draw (power requrements of all connected devices you plug in) you can place on a UPS this is determined by the maximum wattage on the unit. keep in mind that the closer you come to the maximum wattage the shorter your actual back up time will be.

for instance... i am personally using a xs 1300 apc upc which is rated for 780w maximum. my computer can draw anywhere from about 150w to 450w or perhaps a bit higher depending on what i am doing at the moment. playing a very demanding game for instance will draw alot of power. not counting a monitor into the equation i would get about 6-7 minutes of runtime in a loss of power situation. with a monitor reduce this by a minute or two. with the television i currently use i'm looking at around 3-4 minutes at full load of power.

at minimum load i could run about 20-25 minutes.

keep in mind that this is not a cheap ups and has a msrp of about $200. unless you spend an extraordinate amount of money or use some custom car battery system in your basement as a source of power with an inverter and other hardware you will not be able to power a huge amount of equipment.

all the ups is meant for is to plug your pc+monitor into so that when the power goes out you dont damage your pc from a blackout. you then have time to save your work and shut down. it is not meant to keep working for hours and hours unless you have a low power demand system (either a low end desktop or laptop) connected up to it.

some of the cheaper $50-100 are almost completely useless for gaming systems.

i hope this helps you to get an idea how everything works and what you should be looking into.