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Seven years ago a friend gave me a Liebert PowerSure 250 UPS and it has done well for whatever (home) PC and monitor I've plugged into it over the years. I've just ordered a new PC with the Intel core i7-920 and other nice specs for 3 HDDs, a nice graphics card etc, and opted for a 700W power supply.

I'm guessing the new workstation will use a lot more power than my current Shuttle box (this will be a development machine running SQL Server). I found a UPS selector wizard which suggested I should choose a UPS capable of providing 524VA - my current UPS is only capable of 250VA.

I've read good reviews for an APC Back-UPS ES 700VA - which is 700VA, but it's output capacity is only 405 Watts. Does that mean the 700W power supply in my new PC is over-spec'ed? Or have I missed something? I only want the UPS to run for 5 minutes in the event of a power-cut. How do you choose a UPS to cover your power requirements?

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up vote 26 down vote accepted

Obviously, these tools are made by people who stand to make extra profits by skewing the numbers, but I have found they are pretty decent to work with.

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+1 I recently used the APC selector to make a decision. It seemed to take everything relevant into consideration. – Travis Jul 21 '09 at 14:47
Thanks for the links Russ. Probably also fair to say that the website I bought the PC from made extra profits by encouraging me to opt for the 700W PSU. – Dan Jul 24 '09 at 10:56

Some new information for an old thread. Your UPS should be capable of providing the max potential draw of your PSU. I mean, the entire point of the UPS is to protect your data so it should not be the weak link in the chain of components. Also, the price jump to a higher rated UPS isn't that much in the scheme of things.

Another important thing to consider when choosing a UPS is whether your PSU has active power factor correction. These active PFC PSUs are more demanding of their input and can shut down when the UPS kicks on in a power outage. The quick reason - consumer UPSs typically output a squared sine wave approximation and an active PFC supply expects a smooth sine wave. When switching to a stepped wave, they might see a zero power state and just shut off. Not always, maybe sometimes, maybe never depending on the sensitivity of the components... More info on this thread.

As applies specifically to this thread and wattage ratings, a PSU with active power factor correction may actually draw more power when necessary to correct for misaligned current. This is possible anytime as power delivery is never perfect, but much more likely when the current is riding a squared off wave. So while you certainly need to provide for the power demand of all your components, you may need more headroom than you expect if your PSU overdraws to compensate for the output of your UPS.

Better to be safe. Get a UPS that meets or exceeds the wattage of the PSU. Also, buy a UPS that can safely operate with active PFC PSUs (which means the UPS delivers a pure or at least better-than-square wave).

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As a very rough rule of thumb, the wattage rating of a UPS is approximately 0.6 * its VA rating so, as you have seen, a 700VA UPS is good for a power load of around (0.6 * 700) = 420W (your specs said 405W). Conversely, the minimum VA rating you need is approximately 1.6 * load wattage.

Your 700W PSU gives you some idea of how much power your PC will need 'fully loaded', but under normal running conditions it will perhaps use less than half this amount - it's hard to tell without knowing the full specs. And don't forget to add the wattage of your display if you want that to be powered by the UPS too.

Once you have worked out the maximum wattage of your load (the PC, possibly + display), you can choose the base VA rating of UPS you need - then you look at the specs and see which model suits your need. Here's where the fun starts, because you next have to look at how long you want the UPS to run when the power goes - and this can take two paths...

  1. You can pick a UPS that is rated for pretty much the full VA you need so it will be running at 100% of capability and will thus last 'n' minutes.

  2. You can pick a UPS that is rated at a much higher VA value than you really need so, for example, is running at 50% of capability and will thus last for longer than the UPS from option 1.

To complicate matters, you can sometimes buy extra battery packs for use with an 'option 1' UPS so its VA rating is not increased but it will run for longer....

...or you can overspec as per option 2 and know that you can add to the load on the UPS in the future, albeit at the expense of run time.

This is the point where you decide whether to read up on all this and become a UPS choosing guru, or use the manufacturer's UPS selection tools!

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But isn't it true that cooling fans will kick in when the UPS is running at a certain percentage of its rated load capacity (IIRC 80% for APC) in normal online use? Therefore if you go for 'option 1', you will always have the fans running? – paradroid Apr 20 '12 at 2:14

I've read good reviews for an APC Back-UPS ES 700VA - which is 700VA, but it's output capacity is only 405 Watts. Does that mean the 700W power supply in my new PC is over-spec'ed? Or have I missed something? I only want the UPS to run for 5 minutes in the event of a power-cut. How do you choose a UPS to cover your power requirements?

700VA is more than enough for the beefier PCs. Yes, your 700W power supply is probably overspec'd, however keep in mind that 700W is the maximum power that it consumes, not the maximum power it provides... You have 20% overhead for standard power supplies, and 10to 15% for high quality power supplies. That means that your PC components can't use more than ~550W but with 120W for the CPU, 120W for graphics, 18W peak for each disk drive, 50W for the motherboard, 10W per RAM stick, this sums up to a grand total under extreme load of 440W with 8 RAM DIMMs and 4 hard drives.

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A 700W PSU should be capable of deliver 700W. The only question is this its peak performance or it can provide it continuously. Apart from this you've got right - 700W is way to much. – KovBal Jul 21 '09 at 14:53
Thanks for the answer - interesting to see how the ups selector tools probably make their calculation too. – Dan Jul 24 '09 at 10:54
Yes, but 700VA is not the same as 700W – Linker3000 Jan 26 '11 at 20:19
700VA isn't exactly the same as 700W, but it's close enough to assume that it actually is, unless we're talking about big systems like power plants, locomotive engines, etc. Computer power supplies have at the very least 90% amp and volt spikes correlation, so the worst case is 700VA ~ 630W. – wazoox Jan 27 '11 at 8:05

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