Question about Power Supplies

[fourpastmidnight]

First, I want to start off by saying that I've read the excellent post on the issue of choosing a Power Supply on these forums, but I have an additional question as a point of clarification.

I added up all my idle wattages for the system I'm considering building, as well as the "worst-case" scenario of 35W/HDD during drive spin-up upon the server being powered on under the worst-case assumption that they all spin up simultaneously.

The case I'm considering using has support for up to 16 drives, but I'll only be populating 8 of them initially (and for the foreseeable future). So I calculated that my idle load will be about 300W (probably less because I don't know exactly what the case/CPU fans wattages will be when idle) while my max load will be about 521W. I'll be using a Supermicro board with remote management capability, so I don't intend on using any sort of discrete graphics card, which explains the "low" wattages.

So I'm in a bit of a quandary. I figure that with a 650W PSU, my peak load will be 80% utilization, while at idle, it'll be at about 45%. Given this is a NAS, I don't plan on rebooting it often, so I'm not concerned about the 80% startup utilization. I'd often only shutdown my previous NAS once every 2 years or so. I'll also add that I do have a 1500VA CyberPower UPS, in case it matters.

The problem is, most PSUs rated for this wattage don't come with 10 SATA connectors--usually only 6 - 8. And let's ignore expansion considerations. If I go much bigger than 650W, my idle utilization goes below 40% making the PSU much less efficient. So, if I ever want to expand, yes, I may need to upgrade the PSU. Also, I'm (trying) not to go crazy with 80 PLUS Gold vs. 80 PLUS Platinum, etc. I'm doing the math, and unless I can get a Platinum/Titanium for a really good price, the $/W is not in my favor for anything above Gold.

Given I'm not running a discrete GPU, and given that most PSUs in this wattage "class" are single-rail units, I would assume, then, that it'd be safe for me to simply get an adapter/splitter to effectively increase the number of SATA connections that are available for me to use to power the drives, so long as the PSU is of good quality (i.e. not using group regulation). Is that correct?

The last time I built a system was almost 20 years ago, and it was far simpler--grab a PSU and stick it in the system. Computers in those days were much less demanding!

 

[ThreeDee]

If you go with an 80+ Titanium ..then they are efficient at idle as well as under load

 

[Constantin]

There are SATA Splitter cables with built in caps that help distribute the load and reduce spikes on the PSU.

FWIW, my storage-focused NAS consumes about 106Watts at idle with 8 helium-filled HDDs, and a few SSDs.

I second the use of titanium PSUs, not just because of the better part-load performance but also because they tend to be built to a better standard and offer more cables, etc.as part of the package.

the used supermicro 2 or 3U servers are very interesting in this regard as a old chassis paired with a new set of PSUs is a very cost-effective combination of bulletproof construction and low running costs. Last time I looked, Supermicro offered a “quiet” titanium 720W PSU well below the cost of a comparable ATX design.

 

[jgreco]

The problem is, most PSUs rated for this wattage don't come with 10 SATA connectors--usually only 6 - 8. And let's ignore expansion considerations. If I go much bigger than 650W, my idle utilization goes below 40% making the PSU much less efficient. So, if I ever want to expand, yes, I may need to upgrade the PSU. Also, I'm (trying) not to go crazy with 80 PLUS Gold vs. 80 PLUS Platinum, etc. I'm doing the math, and unless I can get a Platinum/Titanium for a really good price, the $/W is not in my favor for anything above Gold.

This is a nonsensical argument. Buying the cheapest PoS PSU you can find because of $/W considerations is like buying a Lamborghini and then going down to WalMart to buy the cheapest discount 12V battery you can find for it. You've already spent, who knows, maybe a thousand dollars on a NAS, and then two or three thousand on hard drives. Cheaping out on the power supply can potentially place that hardware at risk of damage.

I think every discussion I've had where some well-meaning but generally misguided person has come up with this "PSU much less efficient" argument and we've actually looked at numbers has resulted in results that do not strongly favor this argument. Most modern PSU's have a pretty good efficiency curve over a wide utilization percentage, even if not 80plus Titanium, but the thing is, when you look at the difference between options, the opex cost differential is often much less than you might think. I've spent $150-$200 on high efficiency 500W PSU's in the past knowing that I would never recover that massive capex investment from the relatively meager opex savings. OTOH they are still running fine a decade later.

 

[fourpastmidnight]

@jgreco: I'm not going for "cheap", just economical--trying to get the best performance ($/WHr) given the cost of the PSU. I.e. if it's going to take me 10 years to "break even" on the energy savings, then that particular PSU is (probably) not a good buy. So I'm not going for "cheap", I'm going for not overspending on a PSU. I read your excellent PSU selection guide, so I know the perils of "going cheap". But I've also done the research on PSUs and the components and circuitry that goes into them. I'm not an expert by any means, but I do know that anyone can spend $1000+ on a PSU and it could just be a total waste of money. It's not about how expensive it is. It's about how is it built? What's your intended use? What's its efficiency? How much do you pay in electricity such that spending top-dollar on an ultra-efficient power supply means you'll never recoup your extra costs over buying a very similar unit with a lower efficiency rating. Just because a PSU has a lower efficiency rating doesn't mean it's not a good PSU.

However, I do thank other commenters that pointed out that Ti PSUs do not have such a huge efficiency dropoff when there's low utilization. I did forget about that detail when looking at the efficiency charts of the various 80 PLUS standard ratings. That's something to take into consideration, if I can find one for a decent price (again, not "cheap", but a decent price). And, of course, these PSUs are more economical on the larger end of power supplied (and are usually only found on 800W on up). But, due to their efficiency at lower utilization, buying a bigger PSU would allow me to expand in the future and still have plenty of headroom, and the number of SATA connections is not as much of a problem.

@Constantin: Yeah, I decided not to go with a server chasis because this NAS will be in my home office. I would love to have a server chasis, RM or otherwise--but 1) I can't have that much noise since I work from home (I worked with a couple of older Dell PowerEdge 2950 2U servers, as well as a Dell PowerEdge 860 1U--and boy were those things LOUD, not to mention they threw off a LOT of heat); 2) a server chassis quite a bit more expensive (of course, you can find some good deals on eBay, but I'm looking at the SuperMicro X12SPI-TF motherboard); 3) I don't have the room for a server chasis. The 8 disks alone will put off quite a bit of noise, but I had a 5-disk Techus NAS before and it was tolerable. Furthermore, a non-RM case (like a SM one I was looking at) was about $700 (new) and only had 8 bays while the Meshify XL, while not hot-swap, has room for 16 disks + 2 x 2.5" SATA. I'm hoping by going with a non-server case with bigger case fans running at lower RPMs, that I can keep the noise level down to about what I had before with my older Techus NAS (may that piece of junk RIP--but it served its purpose for the 8 years I used it before a disk finally died). I'm not 100% set on this case--but unless I can find a way to have a quiet server case and have it fit in my office, I fear this is a compromise I will need to make with this system.

 

[fourpastmidnight]

@Constantin, well, I re-evaluated my case choices. And, after taking into consideration the cost of a larger, more efficient power supply, it turns out the SuperMicros 743-*-SQ case is not that much more expensive. It's just bigger than I'd like--but you can't beat server quality, hot-swap drive bays, etc. So, thanks mentioning the SM chassis because it caused me to once again critically evaluate my choices.