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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.

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7 Answers 7

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.

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I would add that the greater the VA number the longer your computer can run on battery backup. For each model of APC UPS there is a runtime chart on their website. It will also depend on what other components you have plugged in like a DSL router, USB hub, etc. –  Tim Desjardins Sep 22 '09 at 15:03
A 700w PSU if it were maxed out, would draw more than 700w... Very difficult to do given that most modern PSUs have very specific limitations on all the various rails. –  Brian Knoblauch Sep 22 '09 at 17:25
+1, I've always wondered where PF made it into the equation and why VA != W. In EE school your'e taught that Watts = volts x amps (at least in DC). I'm taking AC Circuit Analysis next semester, so maybe it will make more sense later. –  J. Polfer Dec 16 '09 at 17:08
APC has a white paper on this: apcmedia.com/salestools/SADE-5TNQYF_R0_EN.pdf –  J. Polfer Dec 16 '09 at 17:09
@sheepsimulator - In DC its pretty straightforward because its plain resistive load, but in AC you'll come across Inductive and Capactive loads that'll complicate things a bit, and I suspect you'll understand it better then. And hey learning about it now gives you a headstart for your next sem ;) –  Sathya Dec 16 '09 at 18:05

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).

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I am running dual 20" dell LCD's, a i7 quad core system with a radion 4870 1gb video card, 2 hard drives. –  user3183 Sep 22 '09 at 18:33
The i7 is about 95-130 watts, the radeon 4870 is about 100 more, the lcd monitor (you only need to protect 1) will probably run you 35 watts, and with the hard drives you might be pushing around 300 on peak use. You're probably looking at a 750VA/450W APC on the safe-side although you probably could squeeze into a 550VA/330W. Use a Kill-a-watt to be sure. –  Matias Nino Sep 22 '09 at 18:51
RAM, audio chip, fans, and chipset will add (cumulatively) another 50W, but that still puts you well within the 450W range. –  Joel Coehoorn Aug 14 '10 at 4:08

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.

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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

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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).

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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.

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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:

Because of the way active PFC’s operate, they can sometimes overload the UPS with momentary high inrush current. This can occur when the UPS transfers from online to on-battery operation, creating a momentary loss of power (<8ms). The PFC supply may respond by temporarily drawing an excessive amount of current. Also some PC’s, when awoken from standby (or ‘sleep’) mode, will draw a momentary high inrush current, potentially overloading the UPS if it happens to be running on battery. All APC UPS’s are designed to protect themselves when there is a severe overload while on battery. Some general serverclass UPS’s such as APC’s core Smart-UPS® models will protect itself by actively limiting the overload to a level that it can manage. Other, more economical UPS designs such as Back-UPS® or the Smart-UPS® SC will protect itself by shutting down quickly when it detects a severe overload. This potential for incompatibility should be considered when selecting a UPS – sometimes the most economical choice is not the best one. It is important to note that not all PFC power supplies will cause the UPS overload. However, the incompatibility is most acute in the one of the following situations: • A large server class PFC supply (e.g. rated 500W or more) is used with a Back-UPS or Smart-UPS SC. • The server is equipped with redundant PFC supplies (has two line cords) that are powered by the same UPS. • More than one PFC supply is plugged into the same UPS, bringing the total power rating (nominal) of the power supplies to 500W or more. • A workstation class PC (or high-end gaming PC) is equipped with a PFC power supply rated 500W or more. In any of these situations, APC recommends that a true, pure sine wave, server class UPS be used. Acceptable models include APC’s Smart-UPS®, Smart-UPS® XL and Smart-UPS® RT family of UPSs. However if, a Smart-UPS SC or Back-UPS RS is to be used, the UPS should be sized accordingly.

A critical factor to consider in avoiding an overload trip fault is the ‘nominal’ power rating of the power supplies, not the actual steady state power consumption. For example, a server may have two 600W power supplies in parallel-redundant mode, for a total power rating of 1200W. But the steady state power consumption in this case will be less than 600W. In another example, a high-end workstation with an 850W PFC power supply may only consume 350W under normal operation. So proper sizing of a UPS with active PFC power supplies, to better handle momentary overloads, must take into consideration the maximum power rating of the power supply, not just the actual power consumption of the load. Also keep in mind that if a power supply is rated for 600W output, it’s maximum ‘input’ power will be higher depending on its efficiency. For example, an Energy Star 4.0 compliant power supply has to be more than 80% efficient. That means when it is delivering 600W output power, its input power can be as high as 750W. This ‘input’ power should be the basis for sizing the UPS. Currently not all UPS selectors take these factors into consideration when recommending a proper UPS for servers with active PFC power supplies. Therefore the following guidelines should be followed when recommending a UPS for a PFC load.

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