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 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".
- 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?