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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