Do # of 12V rails matter?

[Fuganater]

So I need to be able to support upto like 20 HDDs. Should I worry about how many 12V rails a PSU has? I know I'm going to have to make my own SATA power cables and I want a modular PSU for that. I can mod each power run right?

[xr4man]

i would say that in your case, most definitely the number of 12v rails would matter.

have you thought of an extra power supply such as this http://www.newegg.com/Product/Produc...82E16817101044

i see that it doesn't have a 5v output though.

[blueonblack]

I've read before that multiple 12V rails were nothing but a marketing gimmick. What matters is total amperage, which is why companies like Corsair have high-end power supplies with single rails and massive amperage output.

Someone please correct me if I'm wrong.

[Fuganater]

So this would be good?

http://www.newegg.com/Product/Produc...82E16817182188

[slaveofconvention]

Under extreme stress, multiple rail PSUs tend to be a little more stable as a ripple in a single rail has less chance of affecting your overall stability. Having said that, if you're pushing a PSU that hard, you have the wrong PSU anyway.

Single rail PSUs have one major advantage over multi-rail ones, and that is that you don't have to consider load balancing at all - again, if you're that close to the threshold you should probably be looking for a different more powerful unit anyway...

[xr4man]

So this would be good?

http://www.newegg.com/Product/Produc...82E16817182188

that would probably do you.

it looks like a 3.5" harddrive pulls .55 amp. so that would be just over 10 amps for your 20 hard drives. plus cpu, mobo, video cards.

[Fuganater]

PSU calc said with all green drives and my hardware I only need 550W.

[xr4man]

ok, so i would go with a 850 or bigger then. you want to keep your power usage around the 60% of power supply capability mark.

[Blibbax]

ok, so i would go with a 850 or bigger then. you want to keep your power usage around the 60% of power supply capability mark.

If he goes with a 550 and the peak, worst case scenario usage of his system is 550, then he won't get close to 60% most of the time. It's very rare for every single component to be stressed to its maximum at one time, and even the CPU and GPU will only reach their TDPs in artificial stress tests.

[x88x]

There are a few exceptions, but the vast majority of modern PSUs are actually single-rail when you look at the actual power conversion. IE, there is only a single 12V power conversion circuit instead of multiple like used to be the case with multi-rail PSUs years ago. The multiple "rails" are really just parallel segments of that single rail that are fused at certain levels. This is useful if, say, something fails and you have a short. It's a lot better to have a 20-30A burst across that short than a 100A. Upshot of that is, you don't really have to worry about PSU rails so much unless you're really going to be pushing it...and like slave said, if you're going to be doing that you should have a more powerful PSU.

As for what specific unit you should buy, assuming 550W meets your needs, I would pick one from this list:
http://www.newegg.com/Product/Produc...e=4097%3A43843
Personally I wouldn't buy a Rosewill unit. I don't know what specifically is inside the one you linked, but Rosewill tend to be rather hit-or-miss. Also, bump up to 80+ Gold.

Specifically, I would go with one of these. Which one, depending on how much you want to spend and how big an upgrade power allowance you want.
Corsair AX850 : 850W : $179.99 ($159.99 after MIR) : review
Corsair AX750 : 750W : $169.99 : review
Seasonic X650 : 650W : $139.99 : review

Or, if you're dead set on a 1000W, this:
OCZ ZX1000 : 1000W : $189.99 : review

The PSU is really not something you want to skimp on, especially when you're powering thousands of dollars worth of hardware. That's not to say you always have to spend lots of money; you can definitely get good PSUs for cheap, but they're few and far between. Rosewill makes sells some good units, but they're really hit-or-miss, depending on who actually made any given model and how well they made it.