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Proper Power Supply Sizing Guidance

Extremely useful information, conservative and safe.
Far better than parroting optimistic manufacturer numbers.
The "suggestion" of using 35 watts per drive is WAY too much. 24 watts is a fairly common number and it can get as low as 5 watts for some 2.5" drives. I spent the time creating a spreadsheet that provides that vendor supplied startup power usage and for the 5400/5900 RPM drives it can go even lower.
The average startup power is 21.81 watts for 62 drives. This is 13.19 watts less then the suggested 35 watts!
Take a look at the Disk Price Analysis spreadsheet in the resource area to get actual numbers.
jgreco
jgreco
As noted in the guide, this is really intended as a tool for people to understand how to size power supplies for smaller systems, generally under 12 drives, because there's a demonstrable history of people failing to allow for spinup current. Once you get to a larger number of drives, there are various other strategies that mitigate, including that your chassis manufacturer has probably supplied a suitable PSU, and that chassis for larger drives tend to correctly take care of issues such as staggered start.

Because it is possible (and common) for drives to be replaced in the future with different drives, it is not a good recommendation to size a NAS PSU for a specific model of drive. You could certainly look at a Xeon D system, note that it takes 60 watts max, then look at a dozen 24-watt-max drives and decide on a 360 watt supply. Cutting it close, but doable. The problem is you might get very regretty when you put in some newer drives that go beyond my 35 watt suggestion, and Biduleohm did identify some that actually exceed this, as I recall.

There is very little harm in having excess capacity available, but if the system browns out while it tries to spin a large number of drives because the PSU is undersized, you can cause damage to the PSU, the drives, and potentially even the system itself. In the end you can do what you like, and you even have a good chance of getting away with it, but if you can't afford to replace your NAS and the data on it, the smart move is probably to size your supply correctly.
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