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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?

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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
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Wow. Thanks! I had no idea a hard drive would use that much power. – Jay Wen Mar 14 '13 at 4:06
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
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 – TheH Nov 20 '15 at 3:49
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 at 20:43

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

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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).

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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

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.

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@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.

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