The drive startups weren't guesses.
I was quoting someone who had posted details about booting. The most power hungry drive tested was the Hitachi which peaked a bit over 15watts during startup. The WD reds were lower. So I was overestimating with 15watts per drive.
I don't typically base product engineering on what some moron at some random web site measured a device taking with a testing rig that might have been no more than a Kill-a-Watt on the entire unit. Quite frankly, the numbers reported there actually look very much like that is actually what was done, if the unit was doing staggered spinup.
Here's what I *can* tell you.
The drive manufacturer specs the
WD30EFRX at 1.73 amps spin current, +/- 10%. That means a possible 1.91 amps. Now the way watts works is you take volts times amps equals watts. 12 volts times 1.91 amps is 22.92 watts.
But now here's the thing. If WD is right and you're wrong, you will be stressing your undersized power supply when it spins the drive, causing the 12 volt rail to sag, and voltage sags during this very important phase can KILL A DRIVE. That peak inrush at 1.91 amps might only be for a fraction of a second, so are you actually sure you want to trust some random web site's measure of the power?
The problem is I'm not sure, aside from testing from the wall, how much power the CPU + mobo + 32GB memory will consume on boot up. It's hard to do that without purchasing the components, and if you get an under-powered PSU there's the whole process of return which I'm trying to avoid.
I read your two threads on the Build/Burn-in and the Hardware one (many thanks again for good information!). Neither detail how one would go about pre-determining how to figure out power consumption for the above components. But maybe I'm being thick and I missed it, this happens often now.
Please refer to the section on
power supplies in this sticky.
You can estimate power consumption for most components by using safe values. The TDP for a CPU gives you an approximate cap for watts burned there. For an E3 board, 20 watts, For a stick of RAM, 5 watts. For an HBA, 15 watts. For a NIC, 5 watts per port. For a fan, look up the manufacturer's specs. Honestly this isn't hard at all. You can get actual watt burn values for specific components (motherboards are a bit of an exception to that) if it actually matters.
So remember that you're looking to run a power supply at maybe 30-40% of its rated design watts, as long as everything else (like spinup current) fits in that envelope comfortably.
The math works out pretty simply. Given a 15 drive chassis, you really need 30 amps at 12v just to spin 15 drives. The base system (the X10 previously quoted, CPU, memory, etc) isn't likely to take more than maybe 150 watts unless you have insane fans or something, plus another 150 watts for 15 drives. So you have a 300 watt base requirement, but your supply also needs to consider that it probably need to be able to provide 40 amps at 12v for the overall system to ensure sufficient power to spin drives, spin fans, AND power the motherboard/CPU.
That usually ends up comfortably fitting into a 650-900 watt supply.
