This question marked as off topic because it's "seeking for hardware shopping recommendations".
The question title itself makes clear this is an incorrect reading ("Calculating an appropriate wattage for my server's new PSU?" - specifically asks about "calculating an appropriate wattage", not what model to buy).
It asks how one can appropriately calculate a suitable wattage that's appropriate for a file server build with a large number of HDDs. That's as much on topic as asking how many cores, how much RAM, what speed processor, or any other "how much is appropriate for these needs" question.
It even says up front, "I don't need a brand recommendation" and "It's the size that needs to be sensible. What PSU size would be "safe" for this build?"
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Can the question be reopened, please?