UPS sizing for server hardware

Dear Experts

We have mounted 2 nos of Dell Poweredge R730 with dual hot plug, redundant power supply (1+1) 750W and one L3 switch and one router to expect 4 hours power backup how much capacity UPS is required i,e 750W * 4 = 3000W and other network equipment's Router, L3 swtich auraba in total 3500W not sure if this how to calculate please suggest how much capacity of UPS required and for the backup of 4 hours and also how much capacity of power socket 15Amps or 20Amps for the power chords to connect.
please suggest
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andyalderStorage OwnerCommented:
You don't double the power just because the server has two PSUs, they are redundant so it runs on just one so it takes 750W max (normally quite a lot less).

Try the APC UPS sizer at

Dell have a more accurate calculator at which takes into account which CPUs, disks and RAM you have.
andyalderStorage OwnerCommented:
Oops, scratch the second link, it's ancient. is for the modern servers.
Dr. KlahnPrincipal Software EngineerCommented:
If you want 4 hour backup, rate the configuration for at least 6 hours.  Nobody maintains UPS batteries properly.  The usual situation is "Ah, leave it alone until the UPS says it can't charge it any more" and when that occurs the runtime may be down to a few minutes.  UPSes  should be periodically tested at the full expected load.
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Mal OsborneAlpha GeekCommented:
750W is the MAXIMUM power a single PSU can manage with a very "loaded" server. You have to have an atypical configuration to get anywhere near that. Most power consuming CPUs, 15K drives in every bay, and high powered GPU cards in every expansion slot.  Likewise, switches tend to give  a "worst case" rating, if they are POE, with all ports powering something.

  Actual consumption for a typically configured 2 socket server is in the region of 200W. A 48 port, managed non POE switch and router combined would maybe pull 100W combined.

Thus, my guess would be around 500W total.  Realistically, to get a proper number you will need to take a measurement.

UPS manufactures tend to quote VA rather than watts, which is bullshit, because that would only apply to a highly reactive load, not something anyone would normally configure.

I would probably look at a UPS of around 1500W. Problem is, that a 1500W UPS will NOT have the battery capacity required to run for 4 hours, typical runtime at a 33% load would be maybe an hour at most. You will need to look at devices that can be expanded with multiple external battery packs, or install a genset. You may need to get a higher output UPS just to get the runtime you want.  4 hours running would mean a LOT of batteries, the more common strategy is to just have enough battery power to shut everything down cleanly.
D_wathiAuthor Commented:
Thanks one and all for the valuable reply, at present I have 2KVA UPS. the newly procured UPS  if connected all the PSU's of both the servers that becomes 750W * 4 and 03 network requirements all put together approx 1850W will this take the load please suggest or we have to go for one more 1 KVA and each one of PSU connect to this 1KVA please suggest.
Mal OsborneAlpha GeekCommented:
2KVA should power everything, assuming typical configurations. A 2KVA UPS, however, is not going to last anywhere near 4 hours. If the UPS has a connector and additional battery packs are available, then adding a few of those might get you there.

A 2KVA UPS is almost certainly "smart", it will come with some sort of utility and a serial or USB connection. You can use the utility to see how much power is being drawn, battery capacity, estimated runtime etc.
andyalderStorage OwnerCommented:
It's better to just run one PSU from each server off the UPS, then if it blows up the other one fed direct from the mains keeps the servers up.

Are these servers/comms supporting remote workers since 4 hours runtime isn't going to be any use to local office staff.

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D_wathiAuthor Commented:
Thanks for the reply, request please help me  understand following 2 scenario

1. if we connect PSU1 of server  to UPS1 and PSU2 of same server to  UPS2 on which power source the server will be running is it through UPS1 or UPS2,  or equally power will be drawn form both UPS  

Similarly if connect PSU1 to UPS1 and PSU2 of same server to direct main source now server will be running on which power source while I understand in total server takes 750W
andyalderStorage OwnerCommented:
Older Dells took the same amount of current from each PSU but the R730 you can set one PSU as primary so it supplies most of the power (and draws the most from the UPS/mains) and the other PSU as secondary so it only takes a tiny bit of power unless the primary fails.
D_wathiAuthor Commented:
thank you very much for the reply, I am really interested to set one PSU as primary let it take most of the power from it and secondary to take less until primary fails, how to configure this can you please give me steps or link please.
andyalderStorage OwnerCommented:
The link is up there ^^
D_wathiAuthor Commented:
thank you so much will go through
David Johnson, CD, MVPRetiredCommented:
for the backup of 4 hours and also how much capacity of power socket 15 Amps or 20 Amps for the power cords to connect.

to be realistic for more than 1 hour you need a generator, in my locale we use natural gas generators which kept our call center online during the 2003 blackout
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