If I have a server whose components are known, and I can look up their active/maximum NOMINAL power draw, how do I get from the total to a UPS VA capacity that will allow it to be used for X amount of time? The problem is that I can think of a load of factors and I don't have much sense which matter and which don't, and how much to allow.

I've brain-dumped all the factors I can think of that might need applying, to show why I'm asking.

I'm trying to understand how each factor matters and their impact, what allowance would therefore be appropriate, and how to do the calculation. (I.e., not just an anecdotal percentage usede by a person at random, or a figure from Google, but something explaining the basis for figures used)

  1. Max power draw requirement might be > average/nominal power draw (perhaps the max power draw will hit a higher level, so we might allow 10% more, or 50W more, or something)
  2. Avoiding pushing UPS to its limit; probably shouldn't draw more than say 70% - 80% of rated output at worst. I assume it won't do harm to draw 100% but it's usually a good idea to overspec power components and not stress them or rely on 100% draw, and might be less reliable, and this allows for defects and UPS product variations.
  3. Power will be lost in the PSU (no PSU is 100% efficient or they wouldn't have fans)
  4. Power factor losses (hopefully small as most PSU's have something like 99%+?)
  5. UPS inefficiency and difference between UPS marketing spec and reality of its capability and duration to power a system at 400+ W
  6. UPS/battery won't be as efficient after 2 - 4 years, or once it's a fair way into a typical battery life
  7. I do not want to make an extra allowance for system growth in future - if it might be enhanced then a new nominal power draw or max prospective draw can be worked out, and I'd allow for the appropriate power draw directly in the original calculation. So I don't need to add a random percentage for a "system growth factor".
  8. The assumption that we might draw full power throughout the shutdown time available might not be true. (Dubious but still). For example if a more typical profile is 60% power draw for all but 2 minutes of the time, and 100%/max for just 2 mins of actually writing out disks and shutting down.
  9. If the UPS provides power to 2 or more servers, it might be that 2 identical servers need sligthtly more than exactly 2x the power draw/capacity. (I can imagine the power provided to 2 PSUs with their own individual loads, in parallel, isn't always exactly twice that of each individual PSU. If that's significant, we might need to add a factor for 2, 3 or more PSUs.)
  10. Type/Quality of PSU output (pure sinewave, whatever)
  11. Any non-linearity in time vs load (for example, perhaps running half the power for twice the time actually needs 2.1 x the VA capacity for some reason?) Maybe UPS output/efficiency isn't exactly linear at all output levels)
  12. Even if a 1500 VA UPS can honestly power a single 750W server for an hour, that's not the same as being able to support 3 x 750W servers for 20 minutes. How is this usually shown in UPS stats?
  13. Conversion factors for W to VA, RMS power, etc. (Any sqrt(2)'s in the mix?)
  14. Anything else?

Obviously many of these factors are ignored because nobody's going to check all of them. Some will be small and aggregated as a single correcting factor.

What if any impact does each of these 12 factors have? How do I realistically get from the nominal power of the server(s) to my required UPS VA capacity? What do I look for in the specs, if I want high output for a short time (say 8 x 400W servers for up to 4 mins shutdown)?


A file server has total max nominal power on paper of 465W. Its detailed spec is:

  • 6 hard drives @ nominal active 10W draw; Motherboard @ (say) 50W draw; CPU @ 140W max TPD; ECC RAM x 8 @ (say) 15W in use; Fans/cooling say 30W; 2 NIC cards @ 30W max power; VGA card @ 5W max; Good quality (silver++) 850W PSU.

There are 3 of these servers, all identical. I want 6 mins uptime after power loss - say 2 mins for power to come back, 2 mins to shut down if it doesn't, and 2 mins safety margin. I'm in the EU so we have 230-240V mains if it's relevant. I'd like a UPS that regulates to a sine wave when power is on, and produces a sine wave when it isn't.

How would I work from the nominal max 3 x 465W draw to a reasonable UPS spec and VA rating?

  • For the sine wave thing you will have to look at the actual UPS systems and how they do their thing. Whenever they are inline or not, whenever they're always active or not and so on. As for the VA requirements. Why would you not base the UPS spec on your nominal max draw? Most of the things you list are things you would need to eyeball or look at the technical specifications for. – Seth Aug 2 '17 at 10:58
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    The thing is, I don't know which of these factors make a difference and which don't, and how much of a difference they make. So it's hard to know how to allow for them. The technical specs are usually manufacturer datasheets and marketing for specific products (not general calcs) or electronic specialists papers and sources (over technical for me, I'm afraid and easily misunderstood). At the same time most online material is dubious, ignoring things that clearly can matter and the calcs seem dubious too. So I'm asking here instead, to get an overview of the issue. – Stilez Aug 2 '17 at 11:35
  • Electrical Engineering – Pimp Juice IT Aug 3 '17 at 3:14
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    If you check the specs of any decent UPS on the manufacturer's web site they will give a simple graph of load vs runtime. You can see roughly how much power your computers use on average with a cheap power meter and probably guess peak requirements based on your hardware. Of course you will soon get less capacity than specified because the batteries degrade over time so best to overprovision a bit. The main thing is not to ensure you will never draw more watts than the UPS is rated at. – James P Aug 3 '17 at 9:29

You're over-thinking this a bit. Even if you went through an exercise like this to calculate exactly how much UPS you need, it would provide "false precision", and carry risks that would undermine the reason for having a UPS.

While many of the component calculations could be exquisitely precise, some of the factors would need to be rough assumptions, and then you would have to throw in some arbitrary allowances. If your assumptions and allowances are off, the UPS could be inadequate in several ways. You could exceed it's power rating, causing it to shutdown when you need it, or the run time could become inadequate to complete a graceful shutdown as the batteries age a little.

Even if it was useful to do the exercise, discussion of all of the factors would far exceed the intended scope of a question and involve subject matter that isn't really on-topic on Super User. However, let me offer a practical and much simpler approach:

  1. Ignore the VA rating. Computers are pretty close to a resistive load. Advertising VA is a way for the UPS manufacturers to post wildly inflated numbers that aren't relevant. What matters is the watt rating.

  2. The computer PSU is already over-rated to provide more margin than you need. Going by the PSU wattage will ensure that you never overload the UPS.

  3. The UPS wattage rating is the maximum power you can pull from the UPS without popping it's breaker. Any UPS in the range you're describing will have a thermal breaker. You don't need to be concerned about damaging the UPS by pushing it to its limit; the concern is not exceeding it's limit and popping the breaker, which would defeat the whole purpose of a UPS (that's equivalent to the blackout that you have the UPS there to protect against).

  4. Unless your PSU has active power factor correction, you don't need a pure sine wave UPS.

  5. Look at the PSU wattage and the wattage of other devices that will be plugged into the UPS battery backup outlets. The devices you want on battery backup are the ones needing to be operational to gracefully shut down -- the computer, monitor, external hard drives, etc. Don't burden those outlets with non-critical energy hogs like printers, speakers, etc.

    Go by the rated wattage of each device. That will be its peak requirement, which will provide a margin of safety against overloading the UPS, and will also serve to buy you a little extra run time. Add up those wattages and get a UPS rated for that or higher. If the total wattage is just a hair over a readily available UPS size, there will be enough cushion that you can use that UPS.

  6. Battery run time is a completely separate consideration. The UPS wattage rating tells you nothing about run time. That's determined by the capacity of the batteries used in the UPS. You need to find the UPS run time specs based on load. There are a lot of UPS units that advertise a high wattage rating but they provide barely enough run time to shut down at that load.

  7. The batteries do degrade over time. That's reflected in run time, not power delivery; it will always deliver up to its rated power. The less load there is on the battery, the more run time you get (but it isn't linear).

    You will have a specific load, and the run time you get from the UPS will decrease as the batteries age. So sizing the UPS based on the listed wattages of what's plugged in will give you an extra cushion. The actual load will be less, which will still allow useful run time as the batteries age.

  8. You don't have to have every item plugged into the same UPS. If you have a lot of equipment, comparison shop a single, high-capacity UPS vs. multiple smaller UPS units.

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