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?
49 Answers
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.
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 WattsUpdate:
Lots of hate in the comments. I encourage commenters to read exactly what this answer says and don't read anything else into it. This answer makes exactly two claims:
1.) The OP's calculations were correct
2.) The OP didn't understand the term "power dissipation".
The answer does not claim that WD specs are correct or that they make sense. In fact I would guess that WD has used the term "power dissipation" incorrectly (as others have already suggested) and should have instead used "average power consumption", but that is just a guess.
Some commenters have mentioned that all energy is lost to heat. This is mostly true, but is technically incorrect because some energy is lost to magnetic and gravitational forces. Since the primary loss of energy is most definitely in the form of heat it does make us raise an eyebrow to WD's supplied specs which do not include any peak or average power consumption numbers (a fact that one commenter for some reason blamed on this answer) and may suggest that the HD in question breaks the law of energy conservation (a fact that another commenter for some reason blamed on this answer).
So again, this answer does two things. It confirms that 12V x 1.78A = 21.36W and it provides a definition for the term power dissipation. That's it!
16If 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).
1Electrical 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 WattsIf 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.
21.36 watts sounds about right. You can use a general rule of thumb for about ~25 watts per 7200 rpm drive.
7@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.
Normal drives need 3-6W when not idle.
High-speed drives (I have a Barracuda) take 8W on average (the documentation says). This means that you need to calculate it as 10W (2W as reserve power).
But that's the avverave power, under "normal" conditions. HOWEVER, according to the same documentation (see link), they can take 30W at spin up. That's only the motor, so don't forget to add 1-2W for the digital part (the microcontroller + circuit board).
When I did the power calculations for my PC, I considered two drives at 30W.
I measured the power consumption of my PC but it was some years ago and I forgot the numbers. I will do it again during gaming and boot.
But I remember that the numbers were not that close to the max power of my PSU (800W I think).
Anyway, the idea is that the power requirements might seem small until you get into one of those "abnormal" running conditions, when all peripherals start to draw the maximum amount of power, at the same time. If you see your computer rebooting "without reason", this might be the cause.
The 1.78A at 12V is a start-up / spin-up peak power. After the drive reaches steady spinning, it consumes quite less, depending on load / seek pattern. That's why serious multi-drive SCSI subsystems have a standard option to turn disk power one-by one, to avoid the surge of current upon power-up.
So, for a 10-disk array, you either need to plan for a 18-20A on 12V rail, or somehow have a built-in option in hardware to turn drive's power in sequence, one-by-one.
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.
