How strong of a psu will I need?

I was wondering what you guys would suggest for a PSU. I'm currently building a unraid server to have 20 hard drives, when I was on the raidz forum one guy told me I'll need atleast 1k but I think he may be wrong as others have said lower. I basically need this to power all the drives, and about 6 120mm fans, the mobo and possibly some blue LED's to make it nice and bright.

I was looking at either one of these PSU's.

AX850

http://www.ncix.com/products/?sku=53719&vpn=CMPSU-850AX%20A&manufacture=Corsair

oR

HX1000

http://www.ncix.com/products/?sku=28797&vpn=CMPSU%2D1000HX&manufacture=Corsair&promoid=1306

I can get the AX850 on sale for $169.69 with a price match but the HX1000 is $220 on sale from $270 ($50 off) with the weekly sale.

Someone on another forum also mentioned I should go with an 80 plus Gold PSU which the AX850 is and the HW1000 is not. What would you guys suggest me doing? And would it be safe you think to toss a few 1 4 pin molex to 2 4 pin molex adapters on the AX850 as I'll need about six-eight extra 4 pin molex's for fans and blue led's. (mainly just a cathode im thinking). So what PSU would you guys recommend? I would like to buy this possibly tonight if possible so I don't miss the AX850 on sale, thanks!

Provided you're going to use Green drives (highly recommended), a Corsair 650W will be sufficient - CORSAIR CMPSU-650TX 650W.  A bargain at $79.99 after MIR

I'll be using a mix of green and hitachi. I'm not sure about the 650 watter though, I know my current pc with 8 drives maxes out my 750watt when I have all 8 copying files at the same time (there WD Blacks) and it will make my psu turn off cause it's drawing to much, I don't want to run into that here with 20 drives.

Ok - fair enough... if you check out Raj's recommended builds (here), you'll see his 650W recommendation.  I'm not sure what your 750W PSU is, but I've been told it's not about the W, but the peak amps that can be delivered.  See this thread.

I think that you'll find the 850W more than sufficient.

I know my current pc with 8 drives maxes out my 750watt when I have all 8 copying files at the same time (there WD Blacks) and it will make my psu turn off cause it's drawing to much...

I'm guessing that it's a multi-rail psu, not a single rail like the Corsairs and most other psus recommended for unRAID.  I suspect that you're simply drawing too much current from a rail which is probably specified at around 20amps.  However, without knowing precisely what model of psu you're talking about, we cannot be sure.

I would certainly recommend an 80+ Gold unit for an unRAID system, which would normally be running 24/7.  I know that they're not the cheapest solution, but I would highly commend the Seasonic products.

I also highly recommend Seasonic products. The Corsair 650TX is made by Seasonic. I am using one with 20+ drives.

The issue of power supply capacity is because hard disks draw a large peak current from the 12 volt rail when spinning up the disks. The power supply needs to be able to deliver enough current to spin up all the disks simultaneously. While the disks are spinning up it also needs to provide power to all of the other components in your system. Don't buy an extra big power supply. You'll waste money buying it and operating it because larger power supplies are less efficient at lower outputs.

Xbitlabs has been performing detailed power measurements of hard disks for a long time now. Here is a quote when they reviewed a bunch of 2T. drives:

Seagate’s HDDs need the highest start-up current (over 2A on the 12V alone) irrespective of the speed the drive’s platters are going to rotate at. They have the lowest 5V consumption, but every other HDD is quite modest in this respect, too. None of them needs more than 1 ampere from this power rail. Western Digital’s products are the most economical overall.

http://www.xbitlabs.com/articles/storage/display/2tb-7200rpm_18.html#sect0

In looking through all of Xbitlabs's tests, the 2.4 A. is the highest I saw, interestingly, the Hitachi 7K2000 that is a five platter 7200 rpm design only needs 1.91 A. current to spin up. WD start currents were even less. So lets do the math.

21 drives x 2.4 A. =  50.4 A. or 605 watts (worst case)

Note that this peak current lasts a very short time, after less than half a second the current has dropped by 10 percent.

http://www.storagereview.com/guide/spinPower.html

All quality power supplies can easily deliver peak currents 10 percent higher than their continuous ratings. As long as the rest of the system does not consume more than 100 watts, a 650 watt, single 12 volt rail power supply is adequate for all combinations of hard disks in a 20+ unRAID server. This is regardless of the rpm, capacity or manufacturer of the hard drive.

@ohlwiler... great analysis - I had never seen the math done before.  Also hadn't appreciated the green vs. not-green unexpected differences.

It's worth adding that some motherboard BIOS settings allow delayed or staggered spin up of hard drives when booting.  Also I have seen some drives that have a link option that delays or inhibits spin up when power is first applied.  In either case they will still spin up normally in response to system commands.  These measures can significantly lower the surge requirements when a large number of drives are used.

Also worth noting that the popular WD green drives do quite well - WD20EARS has a 1.55 amp requirement.  WD also have a software tool to allow a slower spin up to be configured in a drive's BIOS in order to reduce the current further.

data here...

http://www.wdc.com/wdproducts/library/SpecSheet/ENG/2879-701229.pdf

spin up control utility here (I have not tried this myself yet)

http://support.wdc.com/product/download.asp?groupid=609&sid=114〈=en some

The popular Supermicro AOC-SASLP-MV8 cards also allow for staggered spin up groups in their BIOS.

I ended up buying the ax850. It will be able to handle 50 amps on the 12 volt rail, my only complaint is there is not enough 4 pin molex's I had to buy a couple splitters and sata to 4 pin molex adapters.

I ended up buying the ax850. It will be able to handle 50 amps on the 12 volt rail, ...

Good choice - that appears to be identical to the Seasonic X850.  It actually has 70 amp capacity on the 12V rail - I wonder whether this isn't too much, therefore sacrificing some efficiency.

my only complaint is there is not enough 4 pin molex's I had to buy a couple splitters and sata to 4 pin molex adapters.

That was my complaint, too, but not many psus come with more than eight!

It's worth adding that some motherboard BIOS settings allow delayed or staggered spin up of hard drives when booting.

At that point, the power supply must be able to spin up all the drives at exactly the same time.  Using a power supply that is marginal will only cause hair loss... (You'll pull your hair out trying to figure out seemingly unrelated issues on the array)

Joe L.

That's true.  I had forgotten that unRAID may cause all drives to spin up if it is being shut down.