Building a Fast, Energy Efficient NAS box – Part 5

Part 5 – PSU Selection

In this installment of our NAS building project we learn how to choose the right PSU for the job.    We see how to estimate the power needs for an Atom based NAS box, the meaning of 80PLUS certifications and calculate an estimate of the annual electrical cost of running a NAS with a sample array of available PSU.

Atom NAS power usage survey:

My own little NAS box has the following specs:

  • Chembro box with built-in 120W PSU (not certified 80 PLUS)
  • Supermicro D510 based motherboard
  • a WD scorpio blue series 80G 2.5″ hard drive (1-2W power usage)

Plugging in power, video console and networking, boot the machine and see:

  • Peak power usage 22-23W during boot
  • 21W idle
  • Note that the “naked” NAS box actual power usage is a little lower than that since the measurement taken included any amount of power (heat) loss from the PSU unit.

So it’s probably safe to plan for around 20-30W power usage for your base NAS box w/o raid disks connected.  Note:  Live power measurements courtesy of my trusty “P3 Kill A Watt Electricity load meter / monitor” ($17 at Newegg)

Disk power survey:

A power survey (based on spec sheets) of Western Digital, Seagate and Samsung disks (750GB to 3TB):

Read/Write Idle Sleep
2TB WD caviar black 10.7W 8.2W 1.3W
Most other WD disks 6-6.5W 3.7-6W 0.75W
Seagate 9.2W 6.3W ???
Samsung 6-7W 5-6W 1W
WD 2.5″ (160GB) 1.75W 0.8W 0.4W

Furthermore, a quick with my own NAS box suggest that disks can consume 2-3 times their average r/w power usage when spinning up (1-2 second).  Note that it closer to 2 times but I did occasionally see spikes reaching 3 times the average r/w power of my disks.   In summary, we see that today’s 3.5″ SATA-II disk power usage looks like:

  • 6-11W read/write
  • 5-8W idle
  • 0.75-1W  suspend/sleep
  • 12-33W spin up (1-2 sec spike)

How big a PSU do I need?

You should plan your PSU to be able to handle the startup load w/o getting hurt!   For this, review the specs of your prospective PSU and see what they can handle in “peak mode”.    You should aim your idle (with RAID sleeping) and your average read/write to fall in the 20%-80% (or 40%-60%) optimum performance of your PSU.

  • We say that the lowest power consumption you system will see is when the RAID is sleeping:  20-30W.
  • The average power consumption when performing r/w operations:
    • 1 disk RAID:  20-30W base +   6-11W  =  26 – 31W
    • 2 disk RAID:  20-30W base + 12-22W  =  32 – 55W
    • 2 disk RAID:  20-30W base + 18-33W  =  38 – 63W
    • 4 disk RAID:  20-30W base + 24-44W  =  44 – 74W
    • 5 disk RAID:  20-30W base + 30-55W  =  50 – 85W
    • 6 disk RAID:  20-30W base + 36-66W  =  56 – 96W
  • The temporary peak usage at disk spin up and boot time: (2-3x the r/w power usage)
    • 1 disk RAID:  20-30W base + 12-33W    =  32-66W
    • 2 disk RAID:  20-30W base + 24-66W    =  44-96W
    • 2 disk RAID:  20-30W base + 36-99W    =  56-129W
    • 4 disk RAID:  20-30W base + 48-132W  =  68-162W
    • 5 disk RAID:  20-30W base + 60-165W  =  80-195W
    • 6 disk RAID:  20-30W base + 72-198W  =  92-228W

Those numbers seem to line up nicely with the power supplies found on the Rosewill and Sans Digital disk expansion boxes since those boxes do not have a full motherboard, ram and boot disk to run:

  • 4 disk unit:  150W PSU
  • 5 disk unit:  220W PSU
  • 8 disk unit:  300W PSU

According to the 80PLUS standard a PSU has to perform at least 80% efficiency at 20-100% load to obtain an 80 PLUS certification.  The bronze requires 82%, silver 85%, gold 87% and platinum 90%.    In general the sweet spot for best PSU performance (i.e. higher than minimum required efficiency) is to operate between 40-60% of it’s capacity.

Let us look at our sample 4 disk raid system and calculate the percentage of maximum PSU load for sleep/read-write/spin-up on common PSU sizes:

Disk Activity    /  PSU SIZE: 120W 150W 180W 200W 250W 300W
Sleeps: 20-30W 17-25% 13-20% 11-17% 10-15% 8-12% 7- 10%
Read/write: 44-74W 37-62% 29-49% 24-41% 22-37% 18-30% 15-25%
Spin-up:       68-162W probably ok mostly ok ok ok ok ok

This illustrates a bit of a dilemma, maximum power efficiency vs. the ability of our PSU to sustain the spin-up peak power usage with the most power hungry disks on the market.   This can translate simply like this:  If you plan to use the fastest disks available, they will consume more energy so expect that your spin-up maximum power consumption may be on the high end of the range listed above.   If like me, you build a NAS populated with eco/green large disks for large storage (movies/music etc…) and low power usage, then you can expect that your numbers will accordingly be on the lower end of the listed above.

At boot time / disk spin-up my system hits 78-85W with the occasional half a second peek at 100W.   Its 120W built-in PSU is working right in its sweet spot when performing read/write activities and running at nearly 20% load when idle.   A 150W PSU may have been a little “safer” (and still operating in the sweet spot) and a 180-200W would have been completely safe while performing withing the required 80PLUS efficiency range (if not right in the sweet spot).  With a larger PSU (250W/300W), you start to operate below the 80PLUS range (if you have eco/green disks).    Using any of the common PSU found in today’s PCs (450-750W) would result in operating at somewhere between 6-16% load which results in highly inefficient setup (a lot of energy is wasted in heat).

What does the efficiency level mean?

Take a 200W PSU 80PLUS certified PSU (80% efficiency at 20-100% load) and run it a full load (200W).   The unit will provide 200W to the PC and draw another 50W wasted in heat (total draw from the AC outlet: 250W).   So for a NAS box using 70W in normal read/write operations, using a 80PLUS certified PSU where 70 falls somewhere between 20 and 100% of the maximum load, another 17.5W would be wasted in heat (total energy consumption = 87.5W).   If you were to use a larger PSU and have that load fall much below the 20% mark or use a PSU that is not 80PLUS certified or both (worst case scenario!), then you could have a much lower efficiency factor.  A factor of 65% for instance would mean another 38W would be wasted in heat!   A 50% efficiency PSU would waste as much power as it provides to the NAS box (70W!).

A Short Review of Low Power PSU:

Once you start looking for them, you will notice that 80PLUS low power PSUs are not all that common.  The more popular models are 450-600W units for your average PC.    You can find a list of 80PLUS certified PSUs by manufacturer name at [].   Many  computer case come with a PSU already.  You may want to keep or replace the PSU that comes with your case.   Here are a few low power PSU units with a basic estimate of the cost of running a small NAS for one year.


  • The NAS raid disks are active (idle/read/write) for 8hours/day and consumes 65W  (home business, file hosting + movies/music)
  • The RAID disks go in sleep mode (but not the base system so it can still respond to a SAMBA / CIFS request) for 16H/day (21W)
  • The NAS runs 365 days/year
  • Electricy has a base cost of say $0.10/wh  and tier two at $0.20 and tier three at $0.30.   What are those tiers you say?   Here in California (and I assume elsewhere), you are alloted a certain amount of electricity at a base rate.   What energy you consume above that limit is charged at a higher rate (tier 2) and once you go beyond a second set limit, you fall in tier 3 (the real sinners) and they charge you a heftier price (say 3+ x the base rate).   Since we have a hot tub, we are always hitting tier 3 on our electrical bill.  This means that any device I add on top of our normal usage costs me electricity at tier 3 prices.   So for this estimate, I will use an arbitrary rate of $0.30/kwh .
  • If we had a 100% efficiency PSU (now wasted energy), our calculation of cost would be:
    (60W * 8h/day)   * 365 days = 175 KWh / year for active RAID  * 0.30/KWh =   $52/year
    (20W * 16h/day) * 365days  = 117 KWh /year for sleeping RAID * 0.30/KWh = $35/year

Some available low Power PSUs:

sparkle PSU

Pico PSU

Pico PSU Power Adapter


  • picoPSU-120, 120w output, 12v input DC-DC PSU  ($40 at mini-box) OR
  • picoPSU-160-XT, 160W / 200 Watt peak, 12V input DC-DC ATX PSU ($50 at mini-box)
  • WITH a 150W power adapter ($45 at mini-box)
    • Review at
    • Note: THe PicoPSU is completely silent as there is no fan!
    • Chart showing power efficciency at 20W (77.7%) and 65W (87.1%)
    • 87% efficiency at 60W: 175 KWh * (1 + 13/87) * 0.30 / KWh = $60.30/year
    • 77% efficiency at 20W: 117 KWh * (1 + 23/77) * 0.30 / KWh = $45.60/year
    • Yearly Total:  $106
  • Sparkle200W NON certified PSU for ($30 on Amazon)
    • NOT 80Plus certified – this unit claims a maximum efficiency of 65% at 100% load  (implying lower performance at lower load).
    • Assume a best behavior of 65% efficiency (may be generous)
    • Yearly TOTAL: $135
  • SeaSonic SS-300ET Bronze 300W ATX12V V2.3 80 PLUS BRONZE Certified Active PFC PSU – ($42 at Newegg)
    • 80% efficiency at 60W:              175 KWh * (1 + 20/80) * 0.30 / KWh = $67.70/year
    • Assume 67% efficiency at 20W: 117 KWh * (1 + 33/67) * 0.30 / KWh = $52.30/year
    • Yearly TOTAL: $120

As you can see, even at the worst case scenario (third tier electrical rate) working a solid 8 hours/day, the yearly cost difference between the most efficient PSU (the pico PSU and the 300W seasonic) is only $14/year.   It would take 3 years to make up for the difference in price.   In reality, your electrical rate is probably lower than what we used here.

One very important aspect for which we had no specificationss is how much energy do each PSU unit consume while the system is in sleep mode.    As we will review in a later post, it is possible to either shutdown or put the NAS box in sleep mode and drop the system power consumption to < 1 W (reads 0W on the Kill-A-Watt unit).    Then using Wake-On-LAN (WOL) you can remotely via software (could be cron/script based) wake up your NAS box and have it ready for action in as little as 10-20 seconds.    To my great pleasure, the 120W PSU that came with my Chembro case uses virtually no power when there is no load on it (< 1 watt).   The same cannot be said of all PSU and stanby/sleep power usage can be higher (3-15W?).

Choosing your own PSU:

Following the guidelines below will help maximizing your power savings:

  • Most case come with a PSU.   Try it out before spending money on a new unit.
  • Most case take a regular ATX12V size PSU.    Cases that take something different usually come with a PSU.
  • Calculate how much power your system needs.
  • Find the smallest 80Plus PSU that will fit the bill.
  • Shop where they have a good return policy (Newegg/Amazon do).
  • Measure the power usage of your system / PSU in sleep mode and see if you are satisfied with the power consumption knowing that some PSU will deliver a < 1W power usage in sleep mode.


With low power systems, the various power efficiency levels do not yield such a high yearly operational cost difference as they would on more power hungry systems.   Follow the recommended steps to get the best green PSU for your project but at the end of the day,  Don’t sweat it too much on the PSUthe difference in yearly power usage for a a green NAS between the best PSU and an average one won’t be all that big/costly.

In our next post, we will review / survey cases for our NAS system.


About admin

My name is Christian Marcotte. I live in San Diego with my wife Carolyn and our son Jeremiah. This site is a recording of my currrent ponderings, hobbies and interest.
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3 Responses to Building a Fast, Energy Efficient NAS box – Part 5

  1. Rob says:

    This is a helpful review, but you should know that the “80-plus” certification is largely hype. Some PSUs exceed it, while others obtain the rating before official testing. There’s another source of DC power, though. If you get a brick-type PSU along with a DC-to-DC ATX adapter, you can get much higher efficiency.

    If you’ve got most of the hard drives in a RAID bay, the low power rating of the brick PSU won’t matter, anyway. You can expect the system, without internal drives, to consume somewhere around 15w. Another simple way to get a brick power supply is to use the Antec ISK300-65 or ISK300-150, which includes a low-profile bracket.

  2. K. Callis says:

    An outstanding mini-tutorial! I have been kicking around setting up my NAS, and stumbled across your blog when I was searching for 6 bay NAS cases. So I have enjoyed reading your adventures and more importantly, the detail of your finding. As a professional Systems Administrator, I never re-invent the wheel, but instead incorporate someone else’s wheel.

    I am curious about the final deployment. I was planning on using either FreeNAS or Openfiler and call is a day. Are you still going to continue using Ubuntu as the OS of choice. At first I was planning on cramming a lot of functionality in my box. I figured if I did Ubuntu, I could combine the box and make it a firewall/router and a NAS box. But the more I thought about it, I figured it would be more simple to just focus on dedicating a box to act as a NAS, and might as well make it handout off… Hence the thought of using OpenFiler or FreeNAS.

  3. Ben says:

    Thanks for sharing your psu analysis. It’s been very helpful for planning our NAS upgrade!

    PS. We’re former San Diego residents as well, but now reside in PA…

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