This is not as straight forward as it sounds.

Specs from Western Digital's site for a WD 3TB Green Drive:

  • Read/Write 6.00 Watts
  • Idle 5.50 Watts

Looks fine right? Look at this part of the spec: "12 VDC" and "Read/Write 1.78 A".

It was a long time ago, but when I was in college that would mean the drive uses 21.36 Watts (12V x 1.78A). 21.36 Watts is a lot more than the claimed 6.00 Watts.

I want to put four of these in a RAID 10 array, so I want to know the actual max power requirement.

Thoughts? Is this a simple typo? Do I need to plan on ~85 Watts of power to support four drives?

  • That "spec sheet" is a joke and incomplete. There's no mention of current draw for the +5 volt supply. I remember when 5.25" HDDs had full product manuals with graphs for all operations for both voltages. – sawdust Mar 14 '13 at 8:41

Your calculation is correct, but your understanding of the term power dissipation is lacking :)

Electrical Specifications
Current Requirements
    12 VDC
    Read/Write  1.78 A

Power Dissipation <-- Energy measured in watts lost as heat
    Read/Write  6.00 Watts
    Idle        5.50 Watts
    Standby     0.80 Watts
    Sleep       0.80 Watts
  • Wow. Thanks! I had no idea a hard drive would use that much power. – Jay Wen Mar 14 '13 at 4:06
  • 7
    All the energy a hard drive consumes is lost as heat. Every last erg of it. It's not as if you're winding up any springs or lifting any weights! So there is still a disparity between the claimed current requirement and the "power dissipation" numbers. I wonder if the "read/write 1.78A" might be a peak figure and the "power dissipation" figures long-term averages. – Jamie Hanrahan Aug 13 '15 at 5:22
  • 4
    That is incorrect. Some is lost as motion. The disc does weigh something so that is the "lifting weights" part, except the weight is being spun, not lifted. – ubiquibacon Aug 18 '15 at 16:23
  • @ubiquibacon is right, there is some friction that is encountered with the rotational axel the platters are attached to – edhurtig Nov 20 '15 at 3:49
  • 11
    But, other than the stored rotational energy (which won't continually increase unless you continually increase the speed of the drive), all the energy "lost to rotation" is, in fact, converted to heat. Through friction. Which is a process where work/kinetic energy is converted to heat/thermal energy. – jhaagsma Jan 18 '16 at 20:43

Most of you are way off here. You are confusing peak and average draw. 1.78 A is the start-up current value.

When calculating power dissipation, both 5V and 12V are considered. All the power to a drive is converted to heat, with 99% being dissipated by the drive and a small amount dissipated over the interface. Friction in the bearings and airflow on the platters results in heat. Losses in the drive motor electronics and windings and by the chipset waste the rest.

A 6W drive through an 80% efficient power supply will draw about 7.5W at the wall.

21.36 watts sounds about right. You can use a general rule of thumb for about ~25 watts per 7200 rpm drive.

  • 1
    This is absolutely not true. I've got 8x4TB drives, an i3, 16GB of RAM, an SSD, and 6 case fans and my server only consumes 78W at idle at the wall as measured with a kill-a-watt device. During disk load it goes up to ~115W tops. – haventchecked Jan 5 '17 at 18:41
  • 1
    Motors take more power when starting... – Eduard Florinescu May 5 '17 at 12:13
  • 2
    @haventchecked Yes, on average over at least a second or however often your measuring device updates. But for choosing a power supply, you need absolute peak values, even if that peak never shows up on a coarse measurement. Of course you can also just hope that those peaks never happen all at the same time... but that's risky. – Nobody May 9 '17 at 22:11
  • 1
    Considering they all power on at the same time, typically speaking, when the computer is powered on, the peaks WILL happen at the same time with some regularity, and therefore peak values are what need to be used when planning the power requirements of a system. – music2myear May 9 '17 at 22:18
  • Some drives support being started in sequence. e.g. staggered spin-up. This means that the drive will not start spinning until the system (BIOS or EFI) tells it to. Ergo the drives will not all start spinning at the same time IF they support staggered spin-up. – Waxhead Aug 7 '17 at 18:07

If 85W seems like a lot for the PSU you are planning to use for this system - do not forget that during start-up/spin-up the current drain could be almost twice as much (up to 3A per drive).

  • Thanks. This is certainly an eye-opener. I haven't looked at a desktop/server hard-drive since ~2004. I knew I would be looking at more power than a laptop drive, but I didn't know it would be that much more. – Jay Wen Mar 14 '13 at 7:37

@sawdust is correct in that the 5 VDC line isn't truly accounted for...but in most spinning consumer hard drives, the 5 VDC and the 12 VDC lines are about evenly split, and nowhere near 1.75A. For example, the specs on a nine year old WD 500 GB drive (Caviar 16SE WD50000KS) I use in a RAID array are:

5 VDC = 0.70A = 3.5W
12 VDC = 0.75A = 9.0W

The 1.45A here is pretty close to the drive you quoted...but it isn't ALL at 12 VDC, so your calculation is off. This drive draws 12.5W. My array of four drives draws 50W, or 1.2 KwH per day. i.e. my array costs me about $0.20 per day in electricity at local rates.

Green drives may run lower. Newer drives probably run lower. Check both voltages and do the math as above, but my four-drive, ancient array adds about $6 a month to my electric bill. It isn't a HUGE concern. You're more concerned about the Thermal Design Power of your power supply. Always have a far bigger power supply than you need.

One of the arguments for using SSD is that they do use more power but its a smaller average load. This might make all the difference in some setups where you're spinning up the drive regularly. As they can often support DPD mode (essentially sleep) and use system RAM only due to lower latency the battery life can go up. I've tested this experimentally and got maybe a 20% increase with identical system running 10 with Crucial M300 SSD versus 500GB slim 5400 HDD. Its possible to accurately measure this with a tool like "Battmon" or similar and you can actually see the energy usage with things like screen brightness.

Electrical Specifications <-- absolute maximum values

Current Requirements 12 VDC Read/Write 1.78 A

Power Dissipation <-- average operating values

Read/Write  6.00 Watts
Idle        5.50 Watts
Standby     0.80 Watts
Sleep       0.80 Watts

If you start looking at all the appliances around the house, read their electrical specs, then plug them into a power meter and read their actual usage.. it's much lower than what's on the label. Very illuminating.

Your Answer

 

By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.

Not the answer you're looking for? Browse other questions tagged or ask your own question.