12 HDD's take how much PSU ??

2TB Western Digital Caviar Black (WD2001FASS) Specifications:

Transfer Rates: (Serial ATA): 3 Gb/s (Max)
User Sectors Per Drive: 3,907,029,168
Physical Dimensions: 1.03 x 5.79 x 4.00"
Operating Shock (Read): 30G, 2 ms
Non-operating Shock: 300G, 2 ms
(Acoustics) Idle Mode: 29 dBA (average)
(Acoustics) Seek Mode 0: 34 dBA (average)
(Acoustics) Seek Mode 3: 30 dBA (average)
Temperature (English)
Operating: 32° F to 140° F
Non-operating -40° F to 158° F

Power Dissipation
Read/Write: 10.70 Watts
Idle: 8.20 Watts
Standby: 1.30 Watts
Sleep: 1.30 Watts

Try finding a power supply with twenty SATA power connectors on it.

You also need to take into consideration the total start-up current from the +12V and +5V rails when all of the drives start to spin up when the power to the drives is first turned on and you don't have any control over allowing a staggered start-up of each HDD.

2TB Western Digital Caviar Black (WD2001FASS)
Startup Current +12V: 1.16 Amps
Startup Current +5V: 0.60 Amps
Read/Write: 10.70 Watts
Idle: 8.20 Watts
Standby: 1.30 Watts
Sleep: 1.30 Watts

With twenty of these HDDs you can expect a start-up current draw from the +12V rail(s) to be at least 23.2 Amps or greater and from the +5V rail to be 12 Amps or greater.

The power supply needs to be robust enough to handle this inrush current on those rails.

Your bios may have an option to stagger HDD spin-up to save on needing so much extra power just to start your motors.

Awesome Thanks !

I might just add a second PSU.

I have a 1200watt now. My case has enough room for it .

You don't need an additional PSU to power up 20 hard disks.

Use several of these:

4-pin Molex peripheral connector to Dual 15-pin (right-Angled) Serial ATA power adapter

http://www.newegg.com/Product/Product.aspx?Item=N82E168...

I got about 10.... lol

Not accurate enough.