no, the psu can be 1200 watts but rarely systems use 1200 watts from a psu
my ups delivers power to my ps3, my tv and my pc, the ups i have is a cheap model, 650 watts
with all 3 devices working offers around 6 minutes of electricity with a power outage comes
the idea is get a ups with more watts than your devices use
if you pc uses 200 watts, your monitor uses 100 watts and your speakers uses 50, your ups could very well be 500 watts and will do well, for around 8 to 15 minutes, this is not exact but as long as you don't overload the ups maximum power delivery it will not complain, a ups overloaded will beep constantly and will not deliver the charge the devices asks
i like alot apc brand, but there is many brands, usually most people just buy a 650 watts units, remember that the bigger the watts it delver, the bigger and heavier the unit will be, we are talking about kilograms here, a big 2000watts unit can be 20 kilograms so buy one with the most logic size and power delivery that fits your needs
to be very exact on the amount of watts you use and the ups you need you can buy a killawatt and plug each device you want to connect and see how much it uses under load, add the values and you will have a solid figure to know the right size but as i said most people with normal budget gaming pcs and monitors will be happy with a 650 watts psu