How much power supply would I need?

holy crap,maybe a 1000 or 1200 watt psu.

P35+E8500 = Factor in about 150w
16 port RAID card = Factor in about 10w (for the card only)
DVD drive + O/S drive (I'm assuming you'll have one separate from the 20 1TB drives..) = Factor in about 25w
Miscellaneous things (like KB/Mouse/Serial port/etc) = Factor in another 10w
20 x 1TB = Factor in about 400w (20x20) max. However, unless your RAID card supports staggered spin up, you need a PSU with atleast 65-70 amps on the +12v rail (20x3A = 60Amps + 5-10Amps for the CPU and motherboard startup)

All said, you need about 600-700w, BUT you need to size the Amps accordingly, if your raid card does/does not support staggered spin up.

I have a server with Intel dual core @2.66Ghz 4gb ram 8500gt video card, 17 hard drives, and 1 Blu-Ray drive. Its running a SliverStone DA750 that has a single 60amp rail. No problems at all. My raid cards do support staggered spin up im sure that helps with the startup load. I would recommend a brand name PS with the most amps on the 12v rail you can get. Also I have heard that a single rail is better for this use.

For my disk enclosures in my new build, I'm testing a 150w power supply. It's gonna be powering 12 HDDs (but nothing else). We'll see.

Remember that watts are important but so are rails, make sure you get a powersupply with 4+ rails, the Antec 850 TruPower Quattro for example.

For reference, I'm running the following on a Antec Earthwatts 500w power supply:

Supermicro SB7A mobo
Intel Core2d @ 2.67Ghz
8GB ECC DDR2
Adaptec 3450 RAID (hardware, onboard proc)
4x500GB SATA HDD
3x400GB SATA HDD
DVD-Rom
PCI (!!) Video card (it's a server and the onboard crashes 2k8 )

I know it's nowhere close to as many disks as you're talking about, but I figured more info never hurts....

According to PSU Calculator and assuming there will be some RAM and Fans in this box along with a Video Card and Tuner, 716W. I would look at the PC Power and Cooling units, among the most stable with highest single 12V rail PSUs. Their 750W and up models start at 60A 12V rail. Silverstone and a few others also have high end high power single 12V rail supplies, which as pointed out above, would be your best bet.

It's 60 amps peak at 12V, not 120V. 60 x 12 = 720w which is easily handled by an AC circuit even factoring in the power supply inefficiency.

No, that's the DC current, but obviously 750W or so will draw quite a bit of A/C current. @ 110/120V 6.25A A/C to be exact. That's almost half a 15A circuit with no overhead.

Shouldn't that be 240 watts for the drives, not 400? Each drive should be using 12 watts at max, with a idle being around 6 watts. However, for initial spins, if the raid controller does not stagger them, i'd bump up the requirement to 300 watts for the drives, just to give them some head room.
Eitherway, it never hurts to buy a power supply that far exceeds the minimum requirement. And a tip, with that many drives, you should simply be able to spreadout the power load between multiple lines from the psu. Getting a psu with multiple 12v rails may not be a bad idea in this case.

I have my server running off an IStar TC-400R8A 400w redundant power supply. It has 17 hard drives (Areca RAID with staggering enabled) & an LTO2 drive and it hasn't had any issues.

I'm using a 650 watt PC Power and Cooling single rail supply for my Windows Home Server/SageTV server install. I've not actually filled all the drive bays yet (16+1 DVD ROM) but it is rock solid with 10-11 HDDs operating (plus 2 capture cards, 4 pci-e sata cards and a fair number of fans).

All of the recs here seem right, assume 3 amps per drive for the spin up and go for a single rail supply (most high end power supplies cater for heavy current draw from graphics cards so you can find yourself running out of current on the SATA or molex connectors with multi-rail devices).

The other consideration is efficiency at lower power, once the HDDs are up to speed they consume much less power (the current draw for each can drop to less than 1 amp).

From memory storagereview has info on the spin up power reqs of HDDs versus their power reqs when up to speed. There are a lot of PSU review sites, FWIW I prefer jonnyguru.

If the raid card supports staggered spin up (and with 16 ports I would expect it to) you'll be surprised how small a PSU you could get away with. That said assuming equal efficiency a 750 watt psu will consume the same power as a 400 watt psu if they're both feeding a 200 watt load. Efficiency at lower power loads will be what determines your electricity bill over a year.

Yikes. If I ran that many drives in a home system, I would install a SPST NON-momentary toggle switch on one wire of the Molex connector cable leading to each drive (besides the OS drive). Then, I would keep a list on the OS drive of all of the files on each drive, drive A, drive B, etc. I would turn on just what I need. The controller would probably have to be 'hot-swap' compatible unless one were very careful. The electric bill alone would be killer with the 'regular' setup.