Calculating power requirements

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When building a system you need to do some power requirements guesstimates, most often this doesn’t have to be very precise and especially when building a normal form factor PC or server, the power supply is often heavily oversized anyway.

However when trying to build a performant but energy efficient system, things become a little bit more critical. Especially when you really start taking a look at see how much power you are going to need on the 12v and especially 5v rail.

Let’s explore!

Controller (Disk and Network) PCIe/M2 equipment

I will have a dedicated article for this, especially diving into the requirements of older enterprise grade hardware vs newer more efficient hardware.

The CPU

The more energy consuming item in the build will be the CPU. In our case the Ryzen 9 7940HX 16 core mobile chip.

During testing with 2 motherboards I have recorded the following numbers:

BD770i with  AMD Ryzen 9 7745HX
Running Proxmox, HDplex 250w, no disks, no PCIe BiFurcation card, 2x SSD
Idle: 17.5w
Stressed: 122w
Netto power usage CPU max: 104,5w

BD790i SE with AMD Ryzen 9 7940HX
Running Proxmox, HDplex 250w, no disks, no PCIe BiFurcation card, 2x SSD
Idle:
Stressed:
Netto power usage CPU max:

 

So for the CPU alone you need to budget around a 100w of 12v rail power

The HDDs

Here is where things get interesting, 3.5″ HDDs don’t just use 12v but they actually use 12v and 5v. The 5v is often used for the mainboard of the HDD and also it’s actuator while 12v mostly drives the motor spindle.

HDDs draw a massive amount of power when starting up because the motor needs to start the rotation of the spindle from standstill. This is mostly on the 12v rail and once spinning requires a minimal amount of energy to keep going (unless you spin them down).

The 5v rail however will see high usage when there are lots of seek, read or write activities.

Both rails are important to take into account, you can’t just hook up a large amount of disks anymore to modern PSUs because they might now have the 5v rail to support them when things get really busy (such as a ZFS scrub for instance).

For the Seagate Exos X14 14TB drives I’m using in this build their sticker lists the following:

+5VDC 0.90A
+12VDC 0.72A

I did some calculations on that and come up with the following:

Internal 8 bays

External max 2x 3 bays

Total 14 disks combined

5v = 7,2A+5,4A=12,6A x 5v = 63w
12v = 5,76A+4,32A=10,08A x 12v = 120,96w

Combined wattage = 184w

Conclusions total power usage

That’s quite a bit of power that’s needed at maximum usage these HDDs are rated at. It does seem to reflect some real-world numbers I’ve seen though, running a ZFS scrub of the internal 8 disks would result in a power usage increase of 83w! Not quite the 105w max calculated above, but not that far off either.

If we say that the disks in a long time (not startup) scenario can use about 80% of calculated power that means the 184w would be around 150w of real-world usage. That is sustainable with our selected HDplex 250w power supply. We still have plenty of headroom to also have a very busy CPU at the same time.

Overloaded rail with external disks

Although the above conclusion isn’t wrong, it didn’t dive deep enough. If we look at the total power used for the 5v rail the max calculated value is 12,6A while the HDplex 250w can only deliver 10A on it’s 5v rail and depending on how it’s wired up, this might also be driven over a single output connector pin, which is a lot of Amps for a single pin/wire, which will result in voltage drop and that in turn could cause unwanted behavior.

My solution to this is that if the node requires the ability to be able to run external storage bays, I will use a buck-converter going from 12v to 5v to provide the 5v power and not use the internal rail of the HDplex 250w. If you are only going to run the 8 internal disks, the internal rail of the HDplex 250w is plenty and can run this without issue.