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Proper Power Supply Sizing Guidance

I've seen about 1,000 threads like this one where people decide that they can power a dozen hard drives off a 360 watt supply. DO NOT DO THIS.

I've seen another 1,000 threads where people decide to buy the cheapest power supply that they can find. DO NOT DO THIS.

Your NAS lives or dies by its power supply. In building your NAS, you will be putting together a server platform that costs at a minimum many hundreds of dollars, and then putting hundreds or even thousands of dollars of hard drives into it. If your power supply blows, it could potentially ruin some or all of your expensive gear. Do you really want to trust a $16 power supply ?

Meta-Issues To Consider

You've probably come to FreeNAS for its awesome data protection and storage resiliency features. You've hopefully learned that you want server-grade gear, and that you want ECC memory, and that you want redundancy in your storage system. But another question you should ask yourself, how long do you want your storage system to last? Most users are looking to create a storage platform that won't be obsolete next year, and in fact usually want it to last as long as it can.

Your power supply ends up being one of the most complicated bits of electrical engineering in your system. You want to pick a power supply that has high quality components, because a failure of a component could mean anything from power loss to voltage sag to high voltage being fed through to your low voltage computer parts. These are very bad things!

Further, because we want the power supply to work as well in five (or even ten) years as it does today, we have to consider that as components age, their ability to perform slowly degrades. This degradation is made worse if a component is stressed out to near (or past) its specification. In the world of electronics, we typically cope with this using a principle known as derating. This simply means that, for example, if you needed a supply that can deliver 300 watts, you get a 400 watt supply. A typical high quality modern power supply delivers a fairly consistent level of efficiency when loaded between 20%-80% of its rated capacity, so you're not "saving lots of power" by getting a 300 watt supply for a 300 watt load. The rule of thumb in the shop here is that a power supply should never be pushed beyond 80% of its rated capacity.

The Importance of Clean Power

It is important to have a clean source of power, such as that supplied by a UPS. Under normal circumstances, your utility probably provides a fairly clean power, but under certain circumstances, such as a tree branch falling on the feeder, you may experience a brownout, followed by an autorecloser shutting off and then restoring the power to the line (often several times). Each time power returns, you get a significant amount of cruft on the line, from the inrush brownout caused by motors starting (and possibly stalling), etc. Now, you might have "figured" that you can start 12 drives and a Xeon on a 500W PSU, but that's on a normal utility supply. When that power is fluctuating rapidly in an adverse situation, can the PSU still prevail? You definitely want to avoid answering that question. Get a quality UPS as an integral part of your power supply solution.

Calculating How Much Power You Need

The big thing that most people miss when figuring power consumption is drive spinup current. Most drives take up to about 2.1 amps of additional current on the 12 volt rail to start the platters spinning - that's 25 watts per drive. This is in addition to the eight watts that the drive electronics may be consuming. Do note that some drives use less power, and some use more.

<Edit - 10/27/2015 ->
Frequent forum contributor and electronics sorcerer @Bidule0hm did the hard work of actually showing us what the combined 5V+12V power consumption on drives looks like.


Each vertical division is 0.5A. So if we disregard about 1 amp at 5V, it seems clear that there's a massive power suck for about six seconds, peaking around or maybe a little over 2 amps.

I do not particularly like to undercalculate power requirements, because that may lead to voltage sag, which then leads to equipment failure. Some current drives require a lower amount of spinup current (~1.7A) but I encourage you to contemplate that drives might be replaced or upgraded. Running things right on the edge is a bad idea. If you are building a system with more than four drives, I encourage you to look at the specifications for your drives, but still suggest that you want to reserve about 35 watts for each drive.

When spinning up a large fleet of drives (12+), the use of a chassis with a redundant power supply gives you a large extra safety margin. I will note, however, that such an enclosure should still be sized so that it can operate on a single supply.

So there are three things that should ideally be calculated to size your power supply:

1) Total potential watts
2) Total potential 12V amps
3) Average idle watts

The total potential watts is the sum total of all your drives (~35 watts each), ~25 watts for a mainboard, look at the TDP of your CPU for a poor estimation of the peak watts there (~80?), ~6 watts per stick of memory, ~10 watts per LSI 8 port HBA, and ~15-30 watts per fan. Honestly it can be a fair amount of work to come up with a good number, take some time and look at data sheets.

The total potential 12V amps is important because you want to make sure the supply can provide it, and you want to be aware of any other issues, such as if the supply has dual rails.

But also important is the average idle watts. Unlike the calculations above, instead you take the measured or estimated idle watts. For a hard drive, that's often about 6 watts. A modern CPU, might only be 10 watts. Components like the mainboard, memory, and HBA tend to be nearly fixed in their consumption. Fans are a pain in the rear. Just try to see what you can figure out.

Now, look at your total potential watts. It is probably a big number. Multiply that by 1.25, and that's your target size for a power supply.

Multiply that number by 0.20. If the result is less than your average idle watts, you have a power supply size that's going to be in the efficiency sweet spot for a high quality PSU.

Force fitting that into an available PSU is of course a little fun. You are encouraged to go down a shade or up a tier as needed in order to get the right PSU size.

Awww, Do I Have To?

No, you don't. But you really should. However, there are ways to control the amount of load thrown at a PSU. For a NAS, since the thing that normally dominates the calculation is the spinup current for the drives, there are two things:

1) Use a disk with a lower spinup current. I don't like this method, because invariably someone will replace a drive with a higher current unit.

2) Carefully design your systems to use staggered spinup. This is fraught with peril but if you are sufficiently disciplined, can work. On the other hand, you wouldn't be reading this sticky in that case.

The third option:

3) Random chance suggests that drives are not actually LIKELY to all experience a surge inrush at the exact same moment of time. You can tempt fate.

It isn't clear that there's much value to be had in relying on any of these strategies, unless you are driving so many disks that you can't get a sufficiently large PSU.

Power Supply Efficiency/Cost

There's a school of thought that wants to minimize the size of a PSU and instead rely on the manufacturer's competence and build quality. You CAN do this. I think it a fool's errand, but, yes, you can.

The last time I debated this with a supporter of this strategy, I ran some numbers. It turns out that if you run a 60W load on a SeaSonic G-360, efficiency is 86%, and watts consumed at the wall are 70W. If you run the same 60W load on a SeaSonic G-550, efficiency is 83%, and watts consumed at the wall are 72W. That's a two watt difference to move up to a power supply that supplies 50% more power. At a price of 14c/kWh, the additional electrical cost is about $12... over the next five years.

I am totally willing to pay $2 per year more in electricity for a larger supply that is more suitably sized and less likely to ruin my $1000+ NAS boxes. Trying to microsize your PSU is a game for chumps.

Forum Favorite PSU

We don't have much call for non-rackmount gear here in the shop, and I normally don't care to suggest things we don't have a lot of experience with. However, the SeaSonic G-series (360, 450, 550, 650, 750) is a highly respected PSU line that is well-loved by PC builders all around, with high quality components, tight voltage regulation, and a five year warranty. It is also the #1 choice here on the forums.

The G360 is not modular. The others are.

TL; DR - Precalculated Guesses for the Lazy Geek

Okay, so you don't want to mess with all that crap. You just want to know what to buy! The following makes an assumption of a fan for every four drives and a reasonable amount of memory, and an HBA for more than 4 drives. These are ballpark numbers. Guesses based on back-of-a-napkin math. You should do your own homework. And yes of course you're not likely to hit those peak numbers except perhaps during bootup, but they're still a sane thing to shoot for.

1) For an Avoton C2550/C2750 (18-35W board, 12W memory):
  • 1-2 Drives: 132W peak, 46W idle -> SeaSonic G-360
  • 3-4 Drives: 202W peak, 71W idle -> SeaSonic G-360
  • 5-6 Drives: 297W peak, 118W idle -> SeaSonic G-450
  • 7-8 Drives: 367W peak, 134W idle -> SeaSonic G-450
  • 9-10 Drives: 437W peak, 150W idle -> SeaSonic G-550
  • 11-12 Drives: 507W peak, 166W idle -> SeaSonic G-650 or X-650
2) For an E3-1230v3 (32-98W board+CPU, 12W memory):
  • 1-2 Drives: 195W peak, 75W idle -> SeaSonic G-360
  • 3-4 Drives: 265W peak, 91W idle -> SeaSonic G-360
  • 5-6 Drives: 360W peak, 132W idle -> SeaSonic G-450
  • 7-8 Drives: 430W peak, 148W idle -> SeaSonic G-550
  • 9-10 Drives: 510W peak, 174W idle -> SeaSonic G-650 or X-650
  • 11-12 Drives: 585W peak, 195W idle -> SeaSonic G-750 or X-750
3) For an E5-1620v3 (speculative: 70-210W board+CPU, 24W memory):
  • 1-2 Drives: 319W peak, 125W idle -> SeaSonic G-450
  • 3-4 Drives: 389W peak, 141W idle -> SeaSonic G-550
  • 5-6 Drives: 484W peak, 182W idle -> SeaSonic G-650 or X-650
  • 7-8 Drives: 554W peak, 198W idle -> SeaSonic G-750 or X-750
  • 9-10 Drives: 639W peak, 229W idle -> SeaSonic X-850
  • 11-12 Drives: 709W peak, 245W idle -> SeaSonic X-850 or X-1050
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Extremely useful information, conservative and safe.
Far better than parroting optimistic manufacturer numbers.
The "suggestion" of using 35 watts per drive is WAY too much. 24 watts is a fairly common number and it can get as low as 5 watts for some 2.5" drives. I spent the time creating a spreadsheet that provides that vendor supplied startup power usage and for the 5400/5900 RPM drives it can go even lower.
The average startup power is 21.81 watts for 62 drives. This is 13.19 watts less then the suggested 35 watts!
Take a look at the Disk Price Analysis spreadsheet in the resource area to get actual numbers.
As noted in the guide, this is really intended as a tool for people to understand how to size power supplies for smaller systems, generally under 12 drives, because there's a demonstrable history of people failing to allow for spinup current. Once you get to a larger number of drives, there are various other strategies that mitigate, including that your chassis manufacturer has probably supplied a suitable PSU, and that chassis for larger drives tend to correctly take care of issues such as staggered start.

Because it is possible (and common) for drives to be replaced in the future with different drives, it is not a good recommendation to size a NAS PSU for a specific model of drive. You could certainly look at a Xeon D system, note that it takes 60 watts max, then look at a dozen 24-watt-max drives and decide on a 360 watt supply. Cutting it close, but doable. The problem is you might get very regretty when you put in some newer drives that go beyond my 35 watt suggestion, and Biduleohm did identify some that actually exceed this, as I recall.

There is very little harm in having excess capacity available, but if the system browns out while it tries to spin a large number of drives because the PSU is undersized, you can cause damage to the PSU, the drives, and potentially even the system itself. In the end you can do what you like, and you even have a good chance of getting away with it, but if you can't afford to replace your NAS and the data on it, the smart move is probably to size your supply correctly.