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)

- 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)
- 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.
- Power will be lost in the PSU (no PSU is 100% efficient or they wouldn't have fans)
- Power factor losses (hopefully small as most PSU's have something like 99%+?)
- UPS inefficiency and difference between UPS marketing spec and reality of its capability and duration to power a system at 400+ W
- UPS/battery won't be as efficient after 2 - 4 years, or once it's a fair way into a typical battery life
**I do**- 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".*not*want to make an extra allowance for system growth in future- 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.
- 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.) - Type/Quality of PSU output (pure sinewave, whatever)
- 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)
- 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?
- Conversion factors for W to VA, RMS power, etc. (Any sqrt(2)'s in the mix?)
- 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)?

**EXAMPLE TO PLAY WITH**

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?