If my power supply is 700w, do I need a 700w UPS?

You mean an Uninterrupted Power Supply, APC is a manufacturer of Uninterrupted Power Supplies (UPS). A point to note is that the capacity of UPS is mentioned in VA, not in Watts.

1 W = 1 V x 1A x PF where PF= Power factor, a measure of how efficient the conversion is. So a 1kVA UPS = 1000VA * PF, assuming PF = 0.7 (which is about standard) you get 700W, which is sufficient to power a 700W system. However, a 700w PSU will never consume 700W so it is safe to assume that you won't need a UPS of that capacity.

No. Your APC only needs to support the wattage that your computer is actually USING.

I would say most average single PC's out there can be covered by a 300 watt APC. Things like a high end video card, older quad core processors, multiple hard drives, and CRT monitors have the potential to kick the wattage into a higher gear.

APCs get expensive very fast as you up the wattage so if you think you're bordering on the edge, get a Kill-a-Watt (everybody should have one) to measure how much power your PC (with monitor) is using. Make sure you take power readings while running a program that can stress-test all system components (cpu, gpu, hard drives, memory) at once (e.g. Everest Ultimate).

APC has a UPS Selector online that you can use.

Plugging in your information, the selector suggests a 800VA UPS (Best Price), a 1200VA UPS (Best Value) and a 1500VA+ (Best Performance). The "Best Price" one supposedly gives you a runtime of 13 min, "Best Value" = 21 min., and "Best Performance" gives you 50 min.

UPSs are rated in Voltamps, (and use batteries rated in amp-hours )which is almost the same as watts if you dont consider power factor.

Assume your CPU will use 100W max (most made now are 65-95 for Intel). Add in another 75 watts for your motherboard, and another 100 to 400 watts for your gamer video card if you have one (or two, or three or four, or two dual-cards, etc. Check online for power rating). Now add the wattage of your cable/DSL modem, and your router if you have one. Another 50 watts for a wireless card max. Now add about 500 watts (depending on your monitor; CRT, LCD, an size) for your monitor. You want to be able to SEE what you're doing so you can save your work, right? :)

Then add another 10%. Now time yourself. Start writing an email, and set the timer. GO! Now try to save the email, add 10 seconds even if its instant response. Now click START and SHUT DOWN COMPUTER. Once your computer is entirely off, along with the monitor, stop the timer. Now add 10 to 15 seconds or so for your reaction time from saying "CRAP! Power is out!" to actually saving your work.

Mine was almost a minute and a half on Windows XP and a 24" LCD monitor. I have a 1500VA/900W Cyberpower UPS that I got off Amazon for $89 shipped. You'll pay almost as much for a generic PoS from WalMart. My system tested while GAMING lasted 9 minutes, or 16 at idle. It is a line conditioner, too (or make an isolation transformer)

Good software and windows settings that dims your monitor's brightness will make it last even longer when theres an outage.

One thing that KILLS more electronics are LOW voltages, not high. Not often you'll get over 128v RMS in your home, but often you'll get less. (search for BROWN OUTS + ELECTRONICS DAMAGE).

Nobody with a computer, monitor, and modem should mess around with anything less than a 1500VA UPS. I've already thrown away 5 350VA units from APC that lasted under two years (failed, not the battery, either)

I'm a service tech and an electrician & computer geek, so I have a little street cred here :)

Good luck shopping! Online is usually best + no tax.

EDIT: Other things like USB powered devices, that's 400-500mA or so at 5 volts (amps*volts=watts so .5*5=2.5 whee)

Upgrading to a $100 SSD drive and Windows 7 will GREATLY improve your save times, so with a 1500VA device and these upgrades, youll shut down sooner, use slightly less power, and be able to ignore the beeping longer if you REALLY need to shoot some noobs or finish that power cybersex session in Second Life. Power might come on by then, too so you'll not miss a beat. Literally. lol