0PSU LogoThe search for a storage server PSU

I recently pos­ted abi­out my search for a new stor­age serv­er case. My next search involves find­ing a suit­able PSU! It seems in recent years it has got much harder to find the type of hard­ware I like to use, prob­ably because the fash­ion has been to go smal­ler and sleeker which is gen­er­ally more pos­sible in this era of improved per­form­ance-per-watt devices. How­ever, if you have space and you want sys­tems to be cool, quiet, and effi­cient then large is still the way to go.

My cur­rent serv­er has the excel­lent Cor­sair HX520 PSU which I have had for around 10 years. This mod­u­lar PSU has 5 AUX con­nect­ors which can take SATA or Molex cables. I got extra SATA cables and have 1 molex cable and 4 SATA cables which allows me to power both case fans (molex) and up to 16 HDDs (4 x 4 SATA cables). I cur­rently have 12 SATA drives powered with a little bit of spare capacity.

I would be very happy with anoth­er HX520 power sup­ply for my new serv­er but of course being a 10 year old design it is no longer avail­able, and I haven’t been able to find one second hand so I have been search­ing for some­thing equi­val­ent. The prob­lem is that mod­ern designs seem obsessed with provid­ing silly num­bers of GPU power cables (silly giv­en that a sig­ni­fic­ant major­ity of people use either 0 or 1 GPUs, and very few indeed use 2, let alone 3!) but many of them only have 3 AUX con­nect­ors with even the reas­on­ably high power mod­els only offer­ing 4.

It is pos­sible to get mod­els with 5 or more AUX sock­ets but these are typ­ic­ally £200 or more for 1000W or more. My cur­rent serv­er pulls 125W at the sock­et so this sort of PSU would be com­plete overkill and would also be oper­at­ing at only 10% capa­city which is far below the optim­um effi­ciency. A 400W PSU would be far more suit­able (to give head­room for the syst­me under full load and for the drive spin up power draw at boot up).

One final frus­tra­tion is that even if I am able to find a much older mod­el second hand it is likely to be sig­ni­fic­antly less effi­cient that a mod­ern design. With the serv­er run­ning 24.7 effi­ciency does make a dif­fer­ence. Is it too much to ask to find a mod­ern equi­val­ent of the HX520 with an 80 plus gold (or bet­ter) rat­ing for price of around £60?

Bare in mind there is a lot of mis­in­form­a­tion about the amount of power required for sys­tems with mul­tiple HDDs. There is a good thread on Super­User where some typ­ic­al fig­ures are giv­en and a sim­il­ar dis­cus­sion on Tom’s Hard­ware for­ums where a typ­ic­al example is given…

  • WD Cavi­ar Black (WD2001FASS)
  • Star­tup Cur­rent +12V: 1.16 Amps
  • Star­tup Cur­rent +5V: 0.60 Amps
  • Star­tup Power (total): 16.92W
  • Read/Write: 10.70 Watts
  • Idle: 8.20 Watts
  • Standby: 1.30 Watts
  • Sleep: 1.30 Watts

The total spin up power at 17W is fairly typ­ic­al. A WD Red 3Tb uses 15W. 

With 16 of these drives there would be a max­im­um draw of 48W from the 5v line and 223W from the 12V line (271W total). Even with 125W allowed for a CPU under full load and anoth­er 100W for the rest of the sys­tem, the total max­im­um power draw is still only 471W. And let’s remem­ber that the cpu and sys­tem will not be draw­ing max­im­um power at bootup time when the drives are at max­im­um. The final thing to check is that the PSU can sup­ply enough power on the 5V line and 12v line as these each have a max­im­um rat­ing. Even my old Cor­sair HX520 can provide 24A at 5V which is a total of 120W (Basic sec­ond­ary school phys­ics les­sons will tell you that Power = Cur­rent x Voltage) which is 2.5x the demand of this hypo­tet­ic­al 16-drive system.

In fact the area I would have to be care­ful of (if I did­n’t have staggered spin up) is that my old HX520 has 3 12V lines which each have a max­im­um sup­ply of 18A which means 216W. Only 1 of these is used for drive power so if I tried to spin up 16 HDDs sim­ul­tan­eously I would be slightly over my power budget. Of course I do have staggered spin-up and I only have 10 spin­ning-rust based drives so I am well with­in the lim­its. The les­son is clear though — in any build the lim­it­ing factor is more likely to be the max­im­um power provided per 12v line and not the total power the PSU is rated for. Once we know this it becomes appar­ent that there really is no need for a kilo­watt PSU for this sort of sys­tem, but hav­ing staggered spin up is immensely valu­able and even with that, it is worth pay­ing atten­tion to the max­im­um rat­ing of each 12v line, and the max­im­um rat­ing for the 5v sup­ply. Most mod­ern PSUs now use a single 12v line which can sup­ply 50A+ which solves this problem.

It is worth not­ing that using “green” drives that like to go to sleep could cause issues if they all try to spin up at once. If they don’t then you will get a long reponse delay when ever the wake up! I recom­mend using NAS grade (or bet­ter) HDDs for a stor­age sys­tem of this kind. Some good inform­a­tion and graphs about start-up power are avail­able at 45Drives.com

My cur­rent serv­er has 12 drives, 1 raid card, and 2 tv cards. It typ­ic­ally draws about 125W at the sock­et, with all the drives spin­ning and the TV stream­ing. No doubt if I ham­mer the serv­er it will go high­er, but for a glor­i­fied NAS a kilo­watt power sup­ply is utterly overkill. The key thing to watch is the power draw if you don’t have staggered spin-up enabled (which you can prob­ably only enable with a RAID card). I do have staggered spin up, and 2 of my 12 drives are SSDs which don’t have the spin up issue. Assum­ing the new serv­er also draws around 125W then a 500W PSU will be at 25% load, whilst a 1000W PSU will be at 12.5% load. Giv­en an equal 80 plus rat­ing the 500W PSU is almost cer­tain to be more effi­cient at this load, as well as being a reas­on­able price to start with.

I’ve had a good look at the main branded PSU man­u­fac­tur­ers (Cor­sair, Season­ic, EVGA, Antec, FSP, BeQuiet, Sil­ver­stone, Thermal­take, Cool­er­Mas­ter, XFX, Fractal Design, Ener­max) and made the list below of the closest I could find. Most man­u­fac­tur­ers don’t offer any­thing at all below 650W with 5 or more aux con­nect­ors. I’ve included a 750W Cor­sair as I’ve seen it on offer at a sim­il­ar price to 650W mod­els frequently.

Pos­sible new models

  • Cor­sair RM750 (750W; £75; 80+ gold)
  • EVGA Super­NOVA 650 G+ (650W; £85; 80+ gold)
  • EVGA 650 BQ (650W; £74; 80+ bronze)

So far the PSU’s I am aware of to search for second hand are…

  • Cor­sair HX520
  • Cor­sair HX620
  • Cor­sair HX550
  • Cor­sair HX650
  • Season­ic 660XP2
  • Season­ic 650KM3

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