if my power supply is 700w, do I need a 700w UPS?
I am running dual 20" dell LCD's, a i7 quad core system with a radion 4870 1gb video card, 2 hard drives.
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).
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.
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
So, if you run a calculation here, you will find what the actual power consumption of your rig should be.
According to the calculations, mine draws under 250W, (even though it has a 550 W PSU) so I got this 250W PSU.
It is currently working fine for me - its protected from power failures, and isn't overloaded.
I could not connect a monitor to it as well, because that causes it to beep and shut off (ie it detects power draw is over 250 W and gets upset, as its supposed to by design).
There are meters you can buy that measure the amount of AC power being used by a particular wall socket. The one I own is called a "kill-a-watt." You plug the meter into the wall socket, then plug your device into the meter. The meter is then in series in the circuit so it can measure how much current you're drawing. If you use one of these with your computer, you can watch how much power it uses while it boots, and how much it uses while you're web surfing or typing in a word-processor.
What you will probably find is that your computer uses a surge of power while you're booting, but much less after that. This is usually the constraint that forces you to use a power supply with a certain amount of power: if it had less, you wouldn't be able to boot.
If your power supply is 700 W, then the power you actually use while booting should be less than that, and the power you use after booting should be much, much smaller than that in turn. If you really want to know exactly how much you need, you have to use a meter. You can't just calculate it, because power use depends on every device you have connected (hard disk, sound card, ...), and the power they consume varies depending on what the computer is doing.
You only need the UPS to keep you from losing your work, so you don't need it during boot.
Something to keep in mind is the "compatibility" issue that the internet is full of these days. I am referring to an Active PFC PSU and stepped approximation to sinewave UPS.
As I have mentioned in this thread:
For compatibility between an Active PFC PSU and stepped approximation to sinewave UPS, you can find more information in this Application Note by APC. (You can also do a Google search for "APC active PFC application note" should the link expire.)
Here is a summary: